The U.S. role in Europe is uniquely trusted precisely because it is not European and is not part of Europe's old system of rivalries—this external role was the key solution after WWII to both deter Soviet aggression and tamp down intra-European tensions that had ripped Europe apart twice in the quarter-century before 1939, and if the U.S. withdrew from NATO, you could see an erosion of these arrangements and a return to more rivalrous Europe.