There are many apparent reasons to comprehend the isolation of the U.S. among its allies, but McMaster's answer for this is really worth giving an ear to

Regional powers and U.S. allies, nowadays, try to figure out what the nature of this trend is and what it will lead to. Although it is dubbed "pragmatic realism" by actors of this new foreign policy, it raises concern about a new form of isolationism and a more aggressive form of unilateralism. The belated expression of commitment to the collective security clause of NATO has become part of this concern.