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External Support to Insurgencies
Bruce J. Reider
This essay argues that external support to the insurgents is usually a decisive factor in determining the outcome of an insurgency. The first part of this essay looks at historical accounts of specific insurgencies and finds that external support can be a significant factor in determining the outcome. The second part of this essay reviews research on the impact of external support to insurgency, finding that the preponderance of scholars who studied multiple insurgencies came to the same conclusions regarding the importance of external support. First, external support is critical to insurgents. Second, external support can have a decisive impact on the outcome of an insurgency. Third, the presence or absence of external support may indicate the probability of insurgent success. Finally, external support might be the most important factor in an insurgency. This essay concludes that the evidence from historical analysis and scholarly research overwhelmingly demonstrates that external support is a decisive factor in determining the outcome of an insurgency and identifies the need for development of a comprehensive counterinsurgent response theory aimed at isolating the insurgent using the diplomatic, informational, military, and economic instruments of national power.
The United States’ involvement in Afghanistan and Iraq during the previous decade sparked a renewed interest in the study of insurgency and counterinsurgency. For the most part, academics and practitioners focused their attention on revisiting the ideas developed since World War II and put forth by notable counterinsurgency experts such as David Galula, Robert Thompson, Frank Kitson, and Roger Trinquier. These individuals proposed different formulas for counterinsurgency success. The concept of protecting the population and winning “hearts and minds” espoused by Galula and Thompson has particularly influenced counterinsurgency doctrine development in the United States. To a lesser degree, Kitson’s emphasis on intelligence, information and training as the path to defeating insurgency has also been influential. Trinquier’s advocacy of treating insurgents as terrorists and using all means necessary, including torture and physical coercion, has been dismissed as unproductive, immoral and illegal. Unfortunately, none of the aforementioned counterinsurgency theorists emphasize the importance of denying external support to insurgents. Consequently, current United States’ counterinsurgency doctrine includes only cursory reference to insurgent external support, treating outside aid as minor consideration in counterinsurgency operations.
This essay argues that external support to the insurgents is usually a decisive factor in determining the outcome of an insurgency. “External support is a broad term that includes any form of support provided to an insurgent force from outside the political boundaries of the insurgency.” Insurgents can receive active and passive external support. Active support is the intentional provision of sanctuary, logistics, training, political backing, and economic aid. Passive support occurs when an adjoining state is unable to deny access to insurgents. Contiguous borders can facilitate external support while geographic isolation can render external support difficult at best. A review of the literature reveals three distinct categories of writing on the subject of external support to insurgencies. The first category includes historical accounts of individual insurgencies. The second category analyzes collective groups of insurgencies to identify common characteristics. The third category specifically focuses on analyzing the impact of external support on insurgency.
The Historical Record
The first part of this essay looks at what the literature says about specific insurgencies with a focus on whether external support was a factor. A comprehensive review of every insurgency since World War II is beyond the scope of this paper therefore only a representative sample was selected. The following historical accounts illustrate how the narratives of different insurgencies vary according to the most prevalent factors affecting the outcome.
External support was critical to the outcome of the Greek Civil War (1946-1949). Yugoslavia was the primary source of external support to the Communist Party of Greece (KKE). The KKE had access to safe areas and free transit across the border where it established training camps, replacement centers, field hospitals, and supply depots. They also received supplies such as clothing, rations, arms, and ammunition. When Yugoslavia closed its border with Greece in 1949, the KKE was denied a vital sanctuary and logistics source. The Greek government defeated the insurgency soon afterward. Although closing the border was not the only cause of the Communist rout, it had an adverse impact on the insurgency.
Uldarico Baclagon’s detailed history of the Hukbalahap (Huk) Rebellion (1946-1954) describes how the strategically isolated Huks were defeated through a combination of political and social reforms and successful military counterinsurgency operations. External support was not a factor during the insurgency because the geography of the Philippine islands prevented the Huks from receiving outside aid. The insurgency was primarily a peasant movement confined to the rural areas.
During the Malayan Emergency (1948-1960), external support was never a factor either. According to Robert Thompson, “perhaps the greatest advantage of all was that Malaya was completely isolated from outside communist support, having only a 150-mile frontier with friendly Thailand in the north (with whom there was a border agreement under which Malayan police forces could operate across the border) and a 1,000-mile seacoast, which could easily be controlled.” Although the insurgents were able to slip back and forth across the ill-defined border to sanctuary, the Thai government offered no direct external support to the Malayan Communist Party (MCP). Additionally, the MCP only received nominal moral support from China but no material or safe haven.
External support was not a factor in the successful British counterinsurgency in Kenya during the Mau Mau Rebellion (1952-1960). The Mau Mau Rebellion was primarily a tribal revolt led by the Kikuyu population and limited to the central region of Kenya. There is no evidence the Mau Mau received external support from the surrounding British colonies in Uganda and Tanganyika or the Italian colony of Somaliland. Given the Kikuyu’s narrow tribal appeal and Kenya’s geographic location, Andrew Mumford describes the Mau Mau as “perhaps the most isolated insurgent group to fight the British in the post-war era.”
External support was a critical factor during the Algerian Revolution (1954-1962) against the French. The Front de Liberation Nationale (FLN) insurgents received material, financial, diplomatic, propaganda, and moral support from the Communist bloc and Arab countries. Arms and personnel from abroad flowed in through Tunisia and Morocco where the FLN enjoyed sanctuary. The FLN used the difficult terrain and the cover of darkness to their advantage to smuggle automatic rifles, radio sets, light mortars, ammunition into Algeria while sending unarmed recruits across the border to Tunisia for training. In 1957, the French undertook a massive project to seal the borders and deny the FLN external support. In 1958, the French completed construction of the Morice Line along 460 kilometers of the border with Tunisia and 700 kilometers along the border with Morocco. The Morice Line consisted of an electrified fence, barbed wire, mines, and state-of-the-art electronic monitoring devices, which, combined with military patrols and response forces effectively sealed the borders causing cross-border traffic to cease. John Talbott and Alistar Horne estimated closing the borders reduced insurgent infiltration by as much as 90 percent and was one of the major factors that led to the French military victory in Algeria.
Insurgent external support significantly contributed to the defeat of France in Indochina and the United States South Vietnam during the period from 1950 to 1975. Both the Viet Minh insurgents who opposed the French and the Viet Cong who opposed the Americans received extensive external support in the form of supplies, weapons, ammunition, training and sanctuary. Neither the French nor the Americans were ever able to deny the insurgents external support. Between 1966 and 1968, the United States attempted to construct a barrier system to interdict South Vietnam’s borders with North Vietnam and Laos by building the so-called McNamara Line. The McNamara Line consisted of a combination of field fortifications, acoustic and heat-detecting sensors, and aircraft to monitor and disrupt the movement of personnel and vehicles into South Vietnam. The effort was largely unsuccessful and abandoned in 1969.
External support played a pivotal role in determining the outcome of the Dhofar Rebellion (1962-1976) in Oman. The Popular Front for the Liberation of the Occupied Arabian Gulf (PFLOAG) insurgents in Oman received military supplies and training from the Soviet Union and China as well as safe haven and resupply from the People’s Democratic Republic of Yemen (South Yemen). In 1971, the Oman government responded by constructing the Hornbeam Line, a system of fortified outposts, sensors, barbed wire and mines, to successfully restrict movement of personnel and supplies from South Yemen. The Oman government also attempted to diplomatically isolate South Yemen from other Arab states to pressure the South Yemen from supporting the PFLOAG. Without sufficient, prolonged external support, PFLOAG was eventually defeated.
External support figured prominently during the Rhodesian Bush War (1965-1980). In this case, a major source of external support to the insurgents came from the sympathetic Front for the Liberation of Mozambique (FRELIMO). After FRELIMO seized power in Mozambique in 1975, the border with Rhodesia was open to penetration and infiltration. FRELIMO provided logistics and weapons to the insurgents, gave them sanctuary, permitted establishment of training camps, and placed vehicles, railways, and ships at their disposal. Neighboring Zambia, Angola and Tanzania permitted the insurgents to establish active training bases. Botswana served as a transit area to facilitate the movement of recruits and abductees to camps in Angola where Cuban instructors trained them. In 1974, the Rhodesian government began construction on a border minefield obstacle known as Cordon Sanitaire. The project was completed in 1976. The goal of the Cordon Sanitaire was to establish an impassable obstacle to prevent all cross-border movement in the areas where it was erected. However, the Cordon Sanitaire failed to stem the flow of external support to insurgents in Rhodesia because it was not covered by observation or fire, patrolled, or maintained by security forces.
Although J.K. Cilliers acknowledges that while border sanctuaries may be significant factors they are not determinant factors in the outcome of guerrilla wars, he devotes an entire chapter of his book on counterinsurgency in Rhodesia to external support. Cilliers points out that base areas should provide the insurgents a degree of physical security and may be established within the sanctuary of adjacent states. The extent to which bases areas facilitate freedom of movement and security directly influences the impact of external support on insurgent success or failure. From the counterinsurgent’s perspective, an effective border control system must provide detection, delay and neutralization. Key characteristics of an effective border control system are: physical obstacles that delay breaching attempts; alarms that indicate the location of breaching attempts; quick reaction forces that can respond immediately to any alarm; and, constant patrolling and vigilance.
The historical narratives of these insurgencies illustrate that the presence of external support can be a significant factor in determining the outcome of the conflict. Other than the notable exception of the Greek Civil War, scholars generally agree on the facts and interpretations regarding external support in the insurgencies described above. Historical accounts of these specific insurgencies reveal that sanctuary denial, border interdiction and diplomatic pressure have been effective ways to isolate insurgents and deny access to external support but only when adequately resourced and executed. These detailed historical studies of what happened during particular insurgencies enabled other researchers to comparatively analyze multiple insurgencies to discern common characteristics and trends, which is the next topic of this essay.
Common Characteristics of Insurgencies: An Overview of the Scholarly Research
The second category of literature regarding external support contains the articles and books written by scholars who analyzed collective groups of insurgencies to identify common characteristics. This body of literature emerged and became popular in the early 1960s as a consequence of President John F. Kennedy’s interest in guerrilla wars and the United States’ involvement in Vietnam. This paper examines only what various authors said about external support to insurgents in order to identify points of agreement and disagreement.
Even though everyone in this category studied many of the same cases, they did not agree on the importance of external support to insurgents. For example, Chalmers Johnson and James Cross published opposing views with a year of one another. In an article published in 1962, Johnson dismissed the notion that “privileged or active sanctuaries adjacent to the scene of guerrilla activity, and from which the guerrillas draw sustenance, are the key to the problem.” He pointed out that guerrilla victories in China and Cuba did not depend on the insurgents having “unassailable sources of support or refuges within which they were secure.” Although Johnson acknowledged that active sanctuaries “may well have an important effect upon a particular conflict, civilian loyalties within the guerrilla area still determine the effectiveness of such sanctuaries.” Johnson’s assessment of China and Cuba highlights the complexity of insurgencies; the outcome is determined by a multiplicity of factors. In the case of Cuba, the Batista regime was so corrupt that Castro and his followers did not need outside support to win. Conversely, Cross concluded, in his 1963 book, that guerrilla operations “mounted against reasonably responsive and competent governments have little chance of gaining national victory unless they receive sustained and large-scale support across a contiguous border and can look to that border as a sanctuary and base as well as a source of supply.”
Even though counterinsurgency experts such as David Galula and Roger Trinquier did not emphasize the role of external support to insurgents, they did comment on it. Galula wrote that outside support, while not necessary at the start of an insurgency, obviously helps when available. In fact, he considered outside support one of four conditions for a successful insurgency. Galula added that outside support during the middle and latter stages of an insurgency might become a necessity. Disregarding Roger Trinquier’s misguided opinions on the use of torture and physical coercion; his views on external support to insurgents are informative. Trinquier understood the potential impact of external support as a source for insurgent training, reserves of war material, bases from which to stage attacks, command control, and establishment of a provisional insurgent government. External support can provide the insurgent freedom of action and is a factor in determining the duration of the conflict. Consequently, Trinquier found it necessary and indispensible to destroy guerrilla bases abroad.
Robert Thompson, another acknowledged expert on counterinsurgency, merely noted that guerrilla units can cross between one governmental area and another to find sanctuary but he felt it a waste of time and other resources to attempt interdicting the insurgents by building forts or fences along the border. Instead, Thompson recommended targeting insurgent logistics bases.
Charles Wolf generally agreed with Thompson regarding the relative importance of contiguous borders. Wolf found that interdicting logistic support from contiguous border areas, although necessary is not a sufficient condition for counterinsurgency to succeed. According to Wolf,
In all cases where counterinsurgency efforts are generally considered to have been effective, there was either no contiguous land border or the border was substantially closed off. Insurgent movements may succeed in areas that lack a contiguous land border, but they are much more likely to succeed where a contiguous border region provides an easy source of logistic support for the insurgent movement.
J.J. Zasloff reported that his study, published in 1967, did not demonstrate a causal relationship between insurgent success and the presence of external sanctuary. However, he added, “that the intangible elements of external assistance, such as psychological reinforcement, propaganda, and diplomatic advocacy, might be as important as, or more important than, material support.”
Eqbal Ahmad agreed that external sanctuary is more important psychologically and diplomatically than politically or militarily but the value of active sanctuary is not essential to guerrilla success although it should not be underestimated. According to Ahmad, external support can internationalize the insurgent cause.
These perspectives indicate ambivalence toward the role of external support to insurgents. However, other scholars such as John Pustay and John McCuen felt much stronger about the importance of outside aid. Pustay noted that insurgency might be marked by the supply of external aid from friendly Communist states. Pustay’s four solutions to counter external insurgent support included a program of long-range assault on insurgent bases, patrolling, border interdiction, and diplomatic pressure on the involved Communist state in an attempt to force it to terminate its illicit activities. McCuen expressed a stronger view in his classic book, The Art of Counter-Revolutionary War: The Strategy of Counterinsurgency. McCuen wrote, “Seeking outside support must be a vital principle of revolutionary strategy.” “There can be little question as to the importance for the revolutionaries to gain such outside support.”
Thomas Greene’s Comparative Revolutionary Movements was one of the most comprehensive works published during this period. Greene wrote an entire chapter on external support. He was also one of the first researchers to emphasize hypothesis construction and testing to determine the impact of external support to insurgents. Greene identified the extent to which a particular movement receives external support from foreign states and sympathizers as one of the most frequently critical variables in the success or failure of revolutionary movements. Greene’s analysis found that external support often determined not only the success or failure of the revolution but also the very survival of the revolutionary movement.
Anthony James Joes also believed external support for an insurgency can be decisive. “It is virtually impossible to defeat a popular guerrilla army [that has] secure sources of supply and a recovery area.” However, while external support may be a necessary condition for insurgent victory, it does not guarantee success. Additionally, Joes observed that it is not always governments that provide outside support to insurgent movements; diasporas can provide money, recruits, intelligence, and political influence.
Bard O’Neill wrote extensively about external support to insurgents. He realized that popular support rarely provides all the resources insurgents require so they must seek external assistance. When insurgents lack popular support, outside aid from sympathetic countries, other insurgent movements, private institutions in other states, and international organizations is critical. O’Neill defined four categories of external support: moral, political, material, and sanctuary. Sanctuary in contiguous states is preferable while lack of safe haven severely handicaps insurgent movements. A comprehensive assessment must consider every possible factor affecting external support.
Colin Beer found having a sympathetic neighbor that provided active sanctuary, reasonably free from attacks, as a base and supply route for men and materials was a vital aspect of most successful insurgencies. In an article on asymmetric conflict, Ivan Arreguin-Toft wrote that weak actors must have or gain access to the physical or political sanctuary necessary to make insurgency a viable choice.
Other scholars expressed similar perspectives regarding the criticality of external support to the success of an insurgency. According to Erin Marie Simpson, “few insurgencies survive without base areas or foreign sanctuaries.” Ian Beckett studied 74 insurgencies occurring after the end of the Cold War and found that external support initiated, sustained, or assisted 45 of them. He concluded that successful insurgencies require substantial external support. Andrew Mumford argued that external support is not only a critical enabler of insurgent success but also that external support appears to be more effective than popular support. Christopher Paul, Colin P. Clarke and Beth Grill analyzed 30 insurgencies and found that tangible support - the ability of the insurgents to replenish and obtain personnel, material, financing, intelligence and sanctuary – perfectly predicted insurgent success or failure. Even John Nagl, despite his emphasis on adaptation and winning popular support as the keys to success in counterinsurgency, conceded “external support is almost always a prerequisite for a successful insurgency.”
Clearly a preponderance of scholars who studied multiple insurgencies more or less shared the same conclusions regarding the importance of external support. First, external support is critical to insurgents. Second, external support can have a decisive impact on the outcome of an insurgency. Third, the presence or absence of external support may indicate the probability of insurgent success. Finally, external support might be the most important factor in an insurgency.
The Impact of External Support on Insurgency
The third category of literature focuses on research conducted specifically to analyze the impact of external support on insurgency. Rex Brynen was one of the first researchers to specifically focus on the role of sanctuary in an insurgency. He recognized that existing perspectives regarding the importance of sanctuaries were largely descriptive, although almost universally noted, and rarely analyzed in depth. Brynen found that in an earlier study of twelve civil wars of the 1970s, outside actors supported the insurgents in 42% of the cases, representing 83% of the successful insurgencies. He identified four factors that contributed to insurgent use of and reliance on sanctuary states. First was the availability of internal sanctuary. The second factor was the proximity and access offered by potential external sanctuaries. The third factor was the effect of levels of sanctuary support on the choice of sanctuary areas. The fourth factor was the existence of popular support, the willingness of a population to provide recruits and other forms of assistance, within the sanctuary state. Brynen concluded that the loss of external bases or the withdrawal of sanctuary has a devastating, sometimes fatal, impact on the insurgents. His findings confirmed the opinions expressed by earlier scholars about the importance of eternal support.
In 2001, the RAND Corporation published one of the most extensive studies to date on the impact of external support to insurgencies. Authors Daniel Byman, Peter Chalk, Bruce Hoffman, William Rosenau, and David Brannan observed that state support to insurgencies was a common instrument of foreign policy during the Cold War. In contrast, neighboring states tended to provide the majority of external support to insurgents in the post-Cold War period. Additionally, external support from non-state actors such as diasporas, refugees, foreign guerrilla movements, religious organizations, wealthy individuals, and human rights groups increased.
Byman, Chalk, Hoffman, Rosenau, and Brannan assessed post-Cold War trends in external support for insurgent movements using qualitative methods to determine the sources of external support, characteristics and motivations of the supporters, key differences between state and non-state supporters, and what aspects of an insurgency are most and least impacted by outside support. They surveyed 74 insurgencies active since 1991 and discovered that “44 received state support that was judged to be significant or critical to the survival and success of the movement; 21 movements received significant support from refugees; 19 received significant support from diasporas; and 25 gained backing from other outside actors.” They also found that although states remained the most important source of external support to insurgents, non-state actors also contributed significantly. Their research confirmed, “External support for an insurgency can make a movement far more effective, prolong the war, and increase the scale and lethality of the struggle.”
Joseph Celeski also determined that porous borders and sanctuaries can prolong an insurgency by facilitating freedom of movement and providing the insurgents with safe areas for base camps, reconstitution, potential recruits, lines of communication, temporary escape, transnational transit, and training. He recommended attacking the host, sanctuary and border as ways to deny insurgents the critical advantages of external support.
In 2007, the RAND Corporation published a study of counterinsurgency lessons learned from six case studies. Foreign support and sanctuary were two of the variables used in the study by Angel Rabasa, Lesley Anne Warner, Peter Chalk, Ivan Khilko, and Paraag Shulka. They found that in five of the cases the insurgents had sanctuary and received foreign support. Two cases were ongoing insurgencies with the outcome undecided. In one case where there was no sanctuary or foreign support the counterinsurgency was successful. In the remaining three cases, the insurgents won once and the counterinsurgents won twice. Their findings confirmed that sanctuary and foreign support do not guarantee insurgent victory but without sanctuary and foreign support the insurgents are likely to lose.
Paul Staniland focused on the transnational nature of modern insurgencies. Transnational insurgents are not permanent members of the local population; they live and organize in external sanctuaries. Staniland recommended a transnational containment strategy built around three primary elements: border defenses, nationalist propaganda operations, and intelligence cooperation with states where diasporas are located. Thomas Bruscino Jr. also studied the impact of transnational sanctuaries. He conducted a comparative analysis of two case studies: the United States in Vietnam and the Soviet Union in Afghanistan. Bruscino found that external sanctuary was a major factor in both insurgent victories. His overall conclusion was that sanctuaries do not guarantee insurgent victory, nor does the lack of sanctuaries necessarily mean defeat, but insurgents that have access to sanctuaries and outside support generally fare better than those who do not.
A few scholars used quantitative methods to analyze the impact of external support to insurgents. Jason Lyall and Isaiah Wilson III found that external support, particularly safe havens in neighboring states and the provision of military or economic aid, for insurgents increased the probability of state defeat. Using regression analysis of data from 286 insurgencies between 1800 and 2005, they demonstrated that when an insurgent organization received substantial external support the likelihood of state victory decreased by 87 percent.
Idean Salehyan, one of the most prolific researchers on the topic of external support to insurgencies, wrote a book, authored two articles, and coauthored a third article on the subject. He also incorporated quantitative approaches in his research to investigate the role of neighboring states as sanctuary for transnational insurgents. Salehyan’s analysis of time-series cross-section data from 1951 to 1999 showed that access to external bases had a significant effect on the prolongation of conflict.
Salehyan found that the provision of safe havens or sanctuaries was one of the most common types of foreign support for insurgencies; over half of all rebel groups since 1945 (55%) have conducted operations in other countries. He hypothesized that external rebel bases in neighboring territories increase the probability of a militarized interstate dispute between rebel host and home countries and international rivals are less likely to directly use force against one another when transnational rebel bases are located on their territory. Salehyan’s statistical analysis of international conflicts during the latter half of the twentieth century used two variables: external base and foreign intervention. His analysis showed, “the external base indicator is positive and significant, providing strong evidence that rebel sanctuaries are associated with a greater likelihood of interstate conflict” and “the foreign intervention in support of rebels variable is positive and significant, indicating that military and economic support for rebels is also associated with a higher likelihood of international conflict, providing additional evidence that patronage of rebel groups raises the risk of conflict. Furthermore, “In the absence of bases or rivalry, the predicted risk of conflict is rather low, or 2%. External bases and international rivalries independently raise the probability of conflict to roughly 10% and 9%, respectively. The simple additive effect of rivalries and international bases raises the predicted probability of conflict to about 14%.”
Salehyan proposed a theory of transnational rebellion based on two hypotheses. His first hypothesis was that conflicts will endure longer when rebels have access to extraterritorial bases. His second hypothesis was that external rebel bases in neighboring territories increase the probability of a militarized interstate dispute between rebel host and home countries. Salehyan found that the effect of extraterritorial bases on conflict duration was positive and significant. “The availability of extraterritorial bases increases the probability of conflict continuation between 82 and 96 percent” and “extraterritorial sanctuaries have an important substantive impact on how long conflicts last.” Salehyan coauthored a paper with Kristian Skrede Gleditsch, and David Cunningham; the authors found that external support is more likely for moderately strong groups and less likely when rebel groups are very strong or very weak.
Several researchers traced the impact of external support to insurgents by looking at how insurgencies ended. Thomas Mockaitis analyzed twelve insurgencies based on their outcome. He determined that external support to the insurgents was the most important determinant of the outcome in all twelve. Ben Connable and Martin C. Libicki extensively researched 89 case studies using quantitative and qualitative analysis to evaluate the impact of external support to insurgents on the outcome. They determined that sustained and consistent external support to insurgents was a critical factor in deciding the outcome. Not only do insurgencies rarely survive or succeed without some kind of sanctuary but also sanctuary is vital and the availability of sanctuary directly correlates with an increased likelihood of insurgent victory. “Insurgents who have enjoyed sanctuary have won almost half of the conflicts that have been clearly decided (23 out of 52). Only three of the insurgencies that operated without sanctuary ended favorably for the insurgents (three out of 22, with others ongoing or mixed). The total absence of sanctuary leaves insurgents with only a one- in-seven chance of winning (out of decided cases).” “Insurgencies that benefitted from state sponsorship statistically won at a 2:1 ratio out of decided cases. When that sponsorship was wholly withdrawn, the victory ratio for the insurgent fell to 1:4.” Jeffrey Record found that external support “can alter the insurgent-government power ratio.” He concluded that external assistance correlated more consistently with insurgent success than any other explanation and may be the single most important determinant of insurgent war outcomes. According to Record, insurgents are unlikely to defeat only the most incapable governments without external support.
This essay examined external support to insurgents and discovered that a surprising rich body of literature and research has evolved since the conclusion of the Second World War. The literature can be organized into three distinct categories. The first category includes historical accounts of individual insurgencies. The historical narratives reviewed in this paper highlighted the impact of external support on specific insurgencies and provided the factual information necessary for the subsequent phase of scholarly study. This phase produced the second category of literature, which contains analyses of collective groups of insurgencies that identified common characteristics and trends. Among those common characteristics and trends this essay focused explicitly on the portion of the literature that discussed external support to insurgencies. As the field evolved, some scholars narrowly focused their research on analyzing the impact of external support on insurgency. The third category of literature on external support to insurgencies encompasses the published results of specific research on external support.
The evidence overwhelmingly demonstrates that external support is a decisive factor in determining the outcome of an insurgency. The following ten points summarize key findings in the literature. First, external support is vital to insurgents. Second, sanctuary is the most critical form of external support. Third, external support does not guarantee insurgent victory. Fourth, external support can have a decisive impact on the outcome of an insurgency. Fifth, the presence or absence of external support may predict the probability of insurgent success. Sixth, external support is transnational; the most important source of external support is state-sponsored but aid from non-state actors such as diasporas, refugees, other guerrilla movements, and nongovernmental organizations has become increasingly significant. Seventh, external support can prolong an insurgency and increase the level of violence. Eighth, external support to insurgents can increase the likelihood of interstate conflict between neighbors. Ninth, an integrated system of barriers, patrols and reaction forces is the most effective method to physically deny insurgent access to external support across contiguous borders. Tenth, external support might be the most important factor in an insurgency.
As the study of external support to insurgencies continues to evolve, there are several potential directions for future research. The correlation between external support and other factors affecting insurgency needs to be examined. If external support is indeed the most important factor in an insurgency then a comprehensive theory for counterinsurgent response aimed at isolating the insurgent using the diplomatic, informational, military, and economic instruments of national power needs to be developed.
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 Eric Hunter Haas, Operations at the Border: Efforts to Disrupt Insurgent Safe-Havens (Fort Leavenworth, KS: Combat Studies Institute Press, 2011) 1.
 Edward R. Wainhouse, “Guerrilla War in Greece, 1946-49: A Case Study,” Military Review 37, (June 1957): 19.
 Walter Laquer, Guerrilla Warfare: A Historical and Critical Study (Boston, MA: Little, Brown and Company, 1976. Reprint, New Brunswick, NJ: Transaction Publishers, 1998), 284.
 Robert Taber, War of the Flea: The Classic study of Guerrilla Warfare (New York: Brassey’s, 1965. Reprint, Washington, DC: Potomac Books, 2002), 138.
 Robert Thompson, Defeating Communist Insurgency: The Lessons of Malaya and Vietnam (1966. Reprint, St. Petersburg, FL: Hailer Publishing, 2005), 19.
 Andrew Mumford, The Counter-Insurgency Myth: The British Experience of Irregular Warfare (London: Routledge, 2012), 67.
 Peter Brush, “The Story Behind the McNamara Line,” Vietnam (February 1996): 18-24, accessed December 10, 2013. http://msuweb.montclair.edu/~furrg/pbmcnamara.html.
 Stephen A. Cheney, The Insurgency in Oman, 1962-1976 (Marine Corps Command and Staff College, 1984), 30.
 Ibid., 34-35.
 Jim White, “Oman 1965-1976: From Certain Defeat to Decisive Victory,” Small Wars Journal (September 1, 2008): 6, accessed December 6, 2013, http://smallwarsjournal.com/blog/journal/docs-temp/93-white.pdf?q=mag/docs-temp/93-white.pdf.
 J.K. Cilliers, Counter-Insurgency in Rhodesia (London: Croom Helm, 1985), 20.
 Ibid., 174.
 Ibid., 173.
 Ibid., 104.
 Ibid., 652.
 Ibid., 652-653.
 James Eliot Cross, Conflict in the Shadows: The Nature and Politics of Guerrilla War (Garden City, NY: Doubleday & Company, 1963), 10.
 David Galula, Counterinsurgency Warfare: Theory and Practice (New York: Frederick A Praeger, 1964. Reprint, New York: Frederick A Praeger, 2005), 39-42.
 Roger Trinquier, Modern Warfare: A French View of Counterinsurgency (1964. Reprint, Fort Leavenworth, KS: U.S. Army Command and General Staff College, 1985), 97-103.
 Robert Thompson, Defeating Communist Insurgency: The Lessons of Malaya and Vietnam (1966. Reprint, St. Petersburg, FL: Hailer Publishing, 2005), 154.
 Charles Wolf Jr., Insurgency and Counterinsurgency: New Myths and Old Realities (Santa Monica, CA: RAND Corporation, 1965), 20.
 J.J. Zasloff, The Role of Sanctuary in Insurgency: Communist China’s Support to the Vietminh, 1946-1954 (Santa Monica, CA: RAND Corporation, 1967), 80.
 Ibid., viii.
 Gerard Chaliland, ed., Guerrilla Strategies: An Historical Anthology from the Long March to Afghanistan (Los Angeles, CA: University of California Press, 1982), 246-254.
 John S. Pustay, Counterinsurgency Warfare (New York, The Free Press, 1965), 103-104.
 John J. McCuen, The Art of Counter-Revolutionary War: The Strategy of Counterinsurgency (1966. Reprint, St. Petersburg, FL: Hailer Publishing, 2005), 66.
 Ibid., 66.
 Thomas H. Greene, Comparative Revolutionary Movements (Englewood Cliffs, NJ: Prentice-Hall, 1974), 7.
 Ibid., 98-100.
 Anthony James Joes, Resisting Rebellion: The History and Politics of Counterinsurgency (Lexington, KY: The University Press of Kentucky, 2004), 95.
 Ibid., 96.
 Bard E O’Neill, Insurgency & Terrorism: From Revolution to Apocalypse, 2nd ed. (Washington, DC: Potomac Books, 2005), 139-147.
 Colin M. Beer, On Revolutionary War (Great Britain: Galago Publishing, 1990), 46.
 Erin Marie Simpson, “The Perils of Third-Party Counterinsurgency Campaigns,” Ph.D. diss., (Harvard University, 2010).
 Ian Beckett, “The Future of Insurgency,” Small Wars & Insurgencies 16, no. 1 (March 2005): 32, accessed August 6, 2012, http://www.tandfonline.com/doi/pdf/10.1080/0959231042000322549.
 Andrew Mumford, Puncturing the Counterinsurgency Myth: Britain and Irregular Warfare in the Past, Present, and Future (Carlisle, PA: Strategic Studies Institute, 2011), 19.
 Christopher Paul, Colin P. Clarke and Beth Grill, “ Qualitative Comparative Analysis of 30 Insurgencies, 1978-2008,” Military Operations Research 17, no. 2 (2012): 35.
 John A Nagl, Learning to Eat Soup with a Knife: Counterinsurgency Lessons from Malaya and Vietnam (Westport, CT: Praeger, 2002. Reprint, Chicago, IL: The University of Chicago Press, 2005), xvi.
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 Daniel Byman, Peter Chalk, Bruce Hoffman, William Rosenau, and David Brannan, Trends in Outside Support for Insurgent Movements (Santa Monica, CA: RAND Corporation, 2001), 1.
 Ibid., xiv-xviii.
 Ibid., 4.
 Ibid., 2.
 Ibid., 3.
 Joseph D. Celeski, “Attacking Insurgent Space: Sanctuary Denial and Border Interdiction,” Military Review LXXXVI, no. 6 (November-December 2006): 51-57, accessed December 1, 2013, http://usacac.army.mil/CAC2/MilitaryReview/Archives/English/MilitaryReview_20061231_art009.pdf.
 Angel Rabasa, Lesley Anne Warner, Peter Chalk, Ivan Khilko, and Paraag Shulka, Money in the Bank: Lessons Learned from Past Counterinsurgency (COIN) Operations (Santa Monica, CA: RAND Corporation, 2007), xiv.
 Paul Staniland, “Defeating Transnational Insurgencies: The Best Offense Is a Good Fence,” The Washington Quarterly 29, no. 1 (Winter 2005-06): 21-22, accessed June 25, 2012, http://www.tandfonline.com/doi/pdf/10.1162/016366005774859698.
 Thomas A Bruscino, Jr., Out of Bounds: Transnational Sanctuaries in Irregular Warfare (Fort Leavenworth, KS: Combat Studies Institute, 2006), iii.
 Ibid., 8.
 Jason Lyall and Isaiah Wilson III, “Rage Against the Machines: Explaining Outcomes in Counterinsurgency Wars,” International Organization 63 (Winter 2009): 67-106.
 Idean Salehyan, “Transnational Rebels: Neighboring States as Sanctuary for Rebel Groups,” World Politics 59, no. 2 (January 2007): 241, accessed November 21, 2013, http://www.jstor.org/stable/40060187.
 Idean Salehyan, “No Shelter Here: Rebel Sanctuaries and International Conflict,” The Journal of Politics 70, no. 1 (January 2008): 54, accessed November 21, 2013, http://www.jstor.org/stable/30218860.
 Ibid., 57-58.
 Ibid., 60.
 Idean Salehyan, Rebels Without Borders: Transnational Insurgencies in World Politics (Ithaca, NY: Cornell University Press, 2009), 64.
 Ibid., 101.
 Ibid., 84-85
 Ibid., 91.
 Idean Salehyan, Kristian Skrede Gleditsch and David E. Cunningham, “Explaining External Support for Insurgent Groups,” International Organization 65, no. 4 (Fall 2011): 709-744, accessed November 30, 2013, http://www.jstor.org/stable/23016231.
 Thomas R. Mockaitis, Resolving Insurgencies (Carlisle, PA: Strategic Studies Institute, 2011), 5.
 Ben Connable and Martin C. Libicki, How Insurgencies End (Santa Monica, CA: RAND Corporation, 2010), xvii.
 Ibid., 35-36.
 Ibid., xiii.
 Jeffrey Record, “External Assistance: Enabler of Insurgent Success,” Parameters XXXVI, no. 3 (Autumn 2006): 36, accessed November 29, 2013, http://strategicstudiesinstitute.army.mil/pubs/parameters/Articles/06autumn/record.htm.
 Jeffrey Record, Beating Goliath: Why Insurgencies Win (Washington, DC: Potomac Books, 2007).