The posse Comitatus Act expressly prohibits the use of the US Army on US Soil. The Department of the Navy instituted regulations to enforce the Act on the Navy and Marine Corps. The CiC can suspend those regulations and send the Marines to the Border. No violation of the law.
That's not a precise enough statement.
The Posse Comitatus Act prohibits the of the Army on US soil for law enforcement purposes. It does not prohibit the use of the military for Constitutional purposes, such as the Article IV Section 4 protection of states against invasion or domestic violence.
Article IV Section 4The United States shall guarantee to every state in this union a republican form of government, and shall protect each of them against invasion; and on application of the legislature, or of the executive (when the legislature cannot be convened) against domestic violence.
The President could use the Army to secure the southern border from an advancing caravan of foreigners intent on illegally entering the country. If there are any post-mid-term election riots, the President could use the Army to quell the riots because Congress would be in recess, or he can use his Article II Section 3 power to convene Congress immediately into an "extraordinary occasions" session to have Congress formally appeal to the President to deploy the military to quell domestic riots.
-PJ
Isn't there a strip of land alongside the border that is federal land?
Trump just has to declare it a "military zone" and have the troops defend it.