Why do you think he needs a justification? This indicates you believe if he did it without a reason, the military would ignore it or he'd get impeached. That's fantasy. He needs zero real reason. He can just make one up. The shit in DC is all based on a random lie he made.
I don't think he *needs* justification He just usually wants a narrative they can play endlessly on Fox, and all the Republican stooges can have the same talking points Big Balls getting a beat down from a couple of 15-yo girls was good enough for his current action Anything, or *nothing* works
He needed no excuse to effectively put DC under martial law. He just made up an excuse.