So Trump is cleaning up the top layer of the pentagon and replacing them with his loyalists. This is a dictator power grab move whichever way you look at it.

The question is: if Trump orders the Pentagon to start arresting or incapacitating his rivals such as Biden, and top opposition leaders, can Pentagon do that? (I am not asking will they, I am asking can they?)

Can a US President order a military rule unilaterally?

 
Thanks.

Trump has no loyalists left in the military. He can fire all he wants. He's done.

The Posse Comitatus Act essentially forbids the American military from conducting law enforcement activities within the United States.  The National Guard is not included under this ban, those are under the State Governor's command and not part of the Pentagon's chain of command,  (except in war).

There are exceptions: The Insurrection Act, Chapter 13 of Title 10 (10 USC Sections 251-255). This act allows the President to use U.S. military personnel at the request of a state legislature or governor to suppress insurrections. It also allows the president to use federal troops to enforce federal laws when rebellion against the authority of the U.S. makes it impracticable to enforce the laws of the U.S. by the ordinary course of judicial proceedings.

This being said,  the cheeto in the white house can probably find some dupe to do what he wants. But not legally.

get his new minions at DoD to withdraw all troops from Afghanistan, undermining the peace process there,  just so he can say, "I was the one who brought the troops home."
As a practical matter, this could be meaningless, since you can't simply snap your fingers and move thousands of troops. So there may not be enough time for him to do this.