Troops will not carry weapons to the border. We will have 4,000 troops to greet a million or more illegal aliens storming the border. All they will do is paperwork. Sending troops is a PR stunt. They could send 1500 clerks from IRS to do that.
This isn’t the real reason, of course, It’s the excuse. They can go around this as they go around every US law.
This is the reason — the posse comitatus act: