H/T Scott Nestler for pointing out this New York Times piece by William Safire.
... Infantry footwear has been slogging though mud for centuries, but boots on the ground is a relatively new expression. Earliest citation that Matthew Seelinger, chief historian of the Army Historical Foundation, can find is in an April 11, 1980, article in The Christian Science Monitor. During the Iranian hostage crisis, plans for a rescue operation were made in the Carter administration, and there were worries that the Soviet Union would intervene. "Many American strategists now argue that even light, token US land forces - 'getting US combat boots on the ground' " - as the four-star general Volney Warner put it - "would signal to an enemy that the US... can only be dislodged at the risk of war." The vivid figure of speech soon triumphed over the formal "infantry in the field."
More at The New York Times.