@JLeslie You could ask me. 18 USC 1385 says:
Ӥ1385. Use of Army, Navy, Marine Corps, Air Force, and Space Force as posse comitatus
Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army, the Navy, the Marine Corps, the Air Force, or the Space Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.”
As far as I can see, there is no express Act of Congress and this isn’t addressed in the Constitution. The soldiers/sailors/airmen/marines are going down there to help execute the laws of the country. This OUGHT to be an impeachable offense for Biden. Trump sent some active military (a couple hundred) which should have been the impeachable offense for him. His generals stopped him from sending thousands and thousands of troops like he wanted though. But again, the number isn’t the real issue. This law exists to ensure no party in power can just use the military as a tool to take over the country. That is why active military isn’t allowed. National Guard is an entirely different story and they are generally sent at the request of the state governors.