In the United States, a federal statute known as the Posse Comitatus Act, enacted in 1878, forbids the use of the US Army (and, as amended, the US Air Force, Navy, Marine Corps, and Space Force), as a posse comitatus or for law enforcement purposes without the approval of Congress.
See also:
The Posse Comitatus Act in the United States provides that no unit of the United States Armed Forces may, apart from an emergency declaration by the President, enforce civilian law or assist in such enforcement.
True, except under specific conditions only in wartime. Trump will declare war on the invasion when he takes office. When the conditions of the Law of War manual 11.3 are satisfied, NG can be assisted by the Marines on American soil to remove the foreign invaders.
Only the US Congress can declare war. Trump can declare a national emergency and the incoming Congress will likely support him. (Unless Thune stops it.) I'm only pointing out the steps required.
EDIT:
Yes, I misspoke. National Emergency, not War. Results - Use of military on domestic soil.
A law that was set up by the Corporation after 1871. Excrpt when the USMC protected the mail in the Great Depression. Or when the 101st AB helped force integration.
(post is archived)