Re: Fall of empires (and civilization too?)
By law, the end of his term is 20 January 2021, unless he is re-elected. That's why he'll use every legal and illegal trick in the book to win. He'll bribe or threaten members of the electoral collages to vote for him. He'll explain the constitution and the law in his own way, the Department of Justice, the Republicans and parts of the media will support him in that. Possibly the Supreme Court too. George W. Bush got the presidency through the courts instead of the ballot box, Trump can do that too.
But yes, if he can't win then it's unlikely the military and security services will support him any longer. And yes, they will probably win from nut jobs with guns. The resulting chaos would probably be severe enough to be called a civil war, though.
And that is the worrying element. If you map out these elections into a flow chart with five or so different outcomes there is a frightfully high chance of serious disruptions in two or three of them.