Left to his own devices, Trump would undoubtedly withdraw from NATO. Legally, he can do it with a year’s notice. Would he try it in a second term, when he is only minimally accountable to anyone?
He might, but his views are outside of the GOP mainstream in this instance, and the one area in which Republicans in Congress have shown some willingness to break from him is foreign policy. I think you would see a veto-proof majority oppose the withdrawal if it became a reality.
It is more likely that he would just use his powers as commander-in-chief to throw sand in the gears of NATO and watch it come to a halt. Hey, it’s working with the WTO–why not?