Could the European Union fall apart? Would that end NATO, too? Or will the EU’s troubles lead to a more robust defense? “No” to all of the above. To begin, Brexit will not unleash mass defection because a) the 27 remaining nations are far more deeply integrated, and b) they regard the benefit-cost ratio far more favorably than does Britain. At heart an American alliance, NATO will endure, as it has for 70 years, as long as the U.S. guarantee holds. Precisely for that reason, the Europeans will not move toward an autonomous defense. Yet if the U.S. withdraws its umbrella, it is just as likely that Europe would seek safety in accommodating Russia.