The Constitution says "The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States". As supreme military commander, he alone gets to say whether or not to commit troops for such an expedition. Congress does have the power to control the purse. It is a fundamental principle of constitutional law that Congress cannot command the president.

The Constitution says "The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States". As supreme military commander, the president alone gets to say whether or not to commit troops for such an expedition. Congress does have the power to control the purse. It is a fundamental principle of constitutional law that Congress cannot command the president.