If Mexico asked the U.S. Government to guard its southern border, that would be about the sole legal circumstance that could be justified for placing U.S. troops there, unless Mexico first attacked the United States, and the United States went to war with Mexico and sought to stop Guatemala, Belize, Venezuela etc. from sending troops to fight on the Mexican side.
If aggressive, hostile, human flesh-eating aliens debarked from ufo landing craft south of Mexico such as Martians in The War of the Worlds, and began slaughtering their way north, and were in sufficient numbers to be a concern for the United States, Democrats of Congress would debate for a few months sending special investigators and inhuman rights specialists to determine if the illegal Martian aliens were being treated properly and if the Trump administration should be impeached for showing a rude and disrespectful attitude toward the aliens.
A better choice for any U.S. administration that would like to guard Mexico’s southern border might be to cut a deal with Guatemala and Belize to provide free border protection for those nations in exchange for a couple of billion dollars worth of solar panels, agricultural tools, water purification, desalinization devices and so forth annually. Guatemala and Belize may wish to have border control and keep track of who goes north on an entirely legal basis. Coordinating with Mexican immigration officials, non-Mexicans might need an o.k. from Mexico to enter, and Mexico in turn would be accountable if the people they admitted continued on north to attempt illegal entry into the U.S.A.