"To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;"The main duty of the military is to protect us from invasion. Standing guard on the border does just that.
The militia(the National Guard) can be called forth to enforce law per Article I, Section 8, Clause 15.
Woody
The Congress has "provided forth" certain restrictions, including the Posse Comitatus Act. It's not a blank check.