European leaders are seeking to clarify a little-used mutual defence clause in the European Union treaty as questions grow over Washington’s long-term commitment to NATO during a deepening rift with the United States. NATO, founded in 1949, is a military alliance of North American and European countries built on the principle that an attack on one member is an attack on all. But years of tension between Washington under President Donald Trump and its European allies have pushed European governments to place greater emphasis on their own defence capabilities.…