The only President to don a uniform in combat was George Washington:
In reference to the whiskey rebellion:
Their rebellion spread swiftly but they were no match for the power of George Washington. He may have been president at the time, but was not averse to donning his old military uniform and getting back into the fray (imagine today's presidents pulling the ol' uniform out of mothballs and deciding to lead the invasion of Panama, Iraq or Somalia). Washington, with Hamilton by his side, led an army of some 12,000 men into western Pennsylvania and easily put down the revolt. That was the end of the Whiskey Rebellion although, if memory serves, the excise tax was either repealed or (unlike the whiskey) diluted.
http://earlyamerica.com/review/fall96/whiskey.html
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