Ever heard of Posse Comitatus ?
Passed in 1880 to get federal troops to stop bugging the former confederate states.
It does not apply to protecting the border. History proves this especially along the U.S./Mexico border in the early 1900's. In fact, the last (and possibly only) tank battle to be fought on U.S. soil was around that time in Naco, AZ which is just down the road from my house and the scene of much of our current border invasion.
We already have troops on our border in support roles - building fences, etc. We have aerostats run by the Air Force for drug interdiction purposes. And it was only until recent decades that U.S. troops stopped interdicting border crossers.
And besides all of that, the primary purpose of our military - one of its constitutional charters - is to protect our borders. This supersedes any civil war era law.
Comparing protection of our border to domestic law enforcement is like the Democrats thinking the 9/11 attacks were simply a criminal act.