Question

What is the Posse Comitatus Act and how might it limit President Trump's use of military forces to "defend" the U.S.-Mexican border?

Answer 1

The given act prevents the federal government and its president from the use of the military forces to settle civil issues and help civilian authorities unless it is authorized by the US constitution or congress. The act is intended to help protect the rights of the people in civil matters that may not be protected if military forces are used. So, the act prevents authorities from using military forces.

Regarding the US-Mexican border, the problem is the infiltration or illegal immigration  of workers from Mexico to the USA that comes under the civil administration and it should be managed by the civil forces. Hence, president Trump cannot use the military force to defend the illegal immigration via US-Mexican borders.

If there are cases, when military conflict takes place between Mexico and the USA, then president Trump can use military forces to protect the US-Mexican borders.