I have to wonder if he chose the lesser of two evils. Leaving American troops in position to possibly die in a certain turk invasion or moving them to keep your options open in what is going to get messy. We like to think that that handful of Americans and the possibility of their deaths was deterring the turks but I'm not sure that was going to do the job.
There's more to this story that is just going to have to reveal itself as this continues to unfold. Trump did promise to return troops home. Their mission was Isis and was accomplished.
Now the Turks get to deal with it for a while.