Why is the US military not protecting US borders?

Answer 1

The short answer is that the U.S. Military is largely prohibited by U.S. law and military regulations from engaging in operations on U.S. soil, unless under certain conditions.

The Posse Comitatus Act, passed on June 18, 1878 after Reconstruction following the Civil War, specifically limits the powers of the Federal Government from using the military in Law and Order circumstances on non-federal property (i.e., States and local municipalities). In such cases as border problems, which is a law enforcement issue, the individual State governors may use National Guard units. Note that Guard units are not Federal forces until they are actually called up for active Federal service; until that time they are State militias.

The Army and Air Force are specifically prohibited from law enforcement activities on U.S. soil by the Act; the Navy and Marines are specifically prevented by Department of Defense directive, not the Act itself.

The sole exemption is the U.S. Coast Guard, which, although a military force when assigned to the Navy in wartime, is specifically a law enforcement unit. When Navy units are involved in drug interdiction in U.S. waters, there is a Coast Guard officer aboard to direct operations and to actually handle any arrests.

There are certain circumstances where the Act doesn't apply, such as use of the military in national disaster aid, insurrection/rebellion to the point where State authorities have lost the capability of restoring order (Insurrection Act), etc.

The whole purpose of the Act is to make it much more difficult for the President to use the U.S. Armed Forces to assert martial law or intervene in States' law enforcement affairs without proper justification.

HOWEVER...

That does NOT mean that the President won't suspend the act in time of emergencies, though most Presidents have historically used that action sparingly and with good cause. President Clinton did so during the LA riots in the 90's to help restore order. But any President knows that doing so is a slippery political slope; any U.S. forces firing on and killing U.S. civilians has had bad consequences in the past (e.g, Kent State shootings) for the President and his party.