Prevention of using the military to enforce domestic laws, and yes, against citizens. However citizens don't have a tag on their foreheads IDing them as such, so the use of the Natl Guard is preeminent. Having been in the situation, it is sad to see the way the US military has to bend over backwards to prevent offending an illegal because of 'posse comitatus' because you have no way of knowing for sure who the sob is.