The founding fathers thought the different branches would want to keep their power. They didn't expect that feckless senators and representatives would want to hand off parts of their jobs so they could tell voters the bad happenings aren't their fault.
Oh and Israel bombed Qatar with implicit US approval.
He likely does not want to actually invade anyone or do anything that brings actual war to the US but he does want the personal benefits that come with being at war.
https://edition.cnn.com/2025/09/09/americas/venezuela-yvan-g...
What checks to his power?
The US is already in a state of emergency, one magically extended into infinity by the legislative branch redefining what a "day" is. The Alien Enemies act has already been used to justify Trump's mass deportation program as a war against the "invasion" of the US by immigrants, and that continues despite pushback from the courts, because Trump has declared openly that he doesn't consider himself bound by the courts.
And given what he's already gotten away with, his thesis seems to be correct. He isn't going to seek the pretense because he knows he doesn't need to.
But even if those working under him are faultlessly loyal, that doesn't mean the rest of the population is. Trump knows that even his base won't tolerate behaviour that is blatently illegal (such as running for a 3rd term) unless there is some kind of legal justification such as an emergency war situation. This is what I believe this is all in aide of.
So far that's going as well as the Wars on Drugs and Poverty did.