Whether the borders of another nation or our own, we're trained to defend borders, period. In fact, whether advancing phase lines or defending borders, that is precisely what all branches of the U.S. military are trained to do, in "air, land, and sea." These days, Full-Spectrum Dominance delineates it as "terrestrial, aerial, maritime, subterranean, extraterrestrial, psychological, and bio- or cyber-technological warfare."I would fully support our military defending our borders, but they are trained to defend other nations' borders...
That includes borders, anytime, anywhere.
Strike out "foreign" and you have half a winner, as expanding our own borders hasn't been U.S. Policy for many decades. It has always, however, been U.S. policy to protect our own borders, a fact hammered home by 9/11, when all commercial traffic was immediately grounded after determining the threat was aerial in nature and the U.S. Air Force patrolled our own skies, even as our borders and ports were closed, as were all U.S. military bases....or to create new ones as it suits US foreign policy.
Our very own Constitution hammers this point home in Article IV. Section 4. "The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion."
Put simply, the country (United States) protests the states (each of them) against invasion. I doesn't get any clearer than that. Ordinarily, that would be the border patrol. If/when they're overwhelmed, it is the duty of the United States to ramp up defenses, up to and including the United States Military. The Constitution further mentions the Army and the Navy and their duties.
The United States controls the United States military through both Congressional law and by direction of the President, our Commander in Chief (Article II. Section 2.). Including defense of our own borders, or anyone else's, as it suits U.S. policy by direction of our National Command Authorities.