Except American leadership never admits defeat. It takes the NEXT leader who replaces the original to actually reverse the stupid decisions made. In recent conflicts it has taken a number of iterations of leadership change before America has begrudgingly retreated and pulled their forces out, regardless of the costs. (See Iraq, Afghanistan, Vietnam, etc etc)
The idea that Trump, of all people, will weigh the costs and rationally decide to make logical choices now is super optimistic to put it lightly.