When our own military cannot be used to protect our citizens from the border invasion then what’s the use of having them?
There is nothing prohibiting our military from protecting against invasion. Before World War I that was the main use of active duty military.
“Posse Comitatus” act just means the military cannot be used for law enforcement purposes against Americans. American citizens. Setting up a speed trap and issuing tickets, for example.
It is probably true that “our” politicians would never allow the military to be used for its intended purpose, at least if they could help it.