At a meeting earlier this year, top foreign ministry officials from several European countries were asked who among them still believed that the United States would come to their defense if they were attacked. Not a single hand went up. It was a stark indication of the extent to which European governments have concluded that the United States—for so long the backbone of European and NATO security—is no longer a reliable ally. Over the past 18 months, conversations in Brussels and in other NATO capitals have revolved around how to boost the continent’s conventional defense to make up for Washington’s retrenchment. But a quieter and more urgent discussion is also taking place, focused on the status of the ultimate deterrent: the U.S.…