I realize this is kind of a joke, but...
The US will continue to be the most powerful military in history for a very long time even with a highly incompetent top-layer. It will just have less people with the wisdom and power to push back on the president's worst impulses.
Unfortunately(?) there's not enough coke in the world to put much of a dent in our current military spending (which they hope to increase even further to 1.5 trillion dollars in 2027). And if the price of coke ever did become a problem, well the US now believes it reserves the right to the entire western hemisphere which includes Columbia...
On a more serious note there is also likely to be a rapid burst of nuclear proliferation across the globe as everyone else adjusts to this new reality sans the traditional post-WW II world order.
On the current Trump path the world is going to get far more dangerous and chaotic, not less.
I get the impression that Trump is pretty negative on nuclear weapons and I don't think he'd do something that could provoke nuclear retaliation. But I doubt he'll be our last mad king. I think the odds are pretty high of at least a small nuclear war within my lifetime. Even if the US keeps it together, proliferation means much higher odds of some idiot leader somewhere pressing the Button.