CAEN, France — For almost 80 years, the United States and Europe told themselves that a lasting postwar peace had been won here, in the bloody shallows and sands along 6 miles of Normandy beach. On Thursday — the 80th anniversary of D-Day, perhaps the last major milestone for many of the invasion’s dwindling heroes — that peace in Europe finds itself shattered. Replacing it are fears of another world war that until recently seemed outlandish, uncertainty about Washington’s European allyship that the Normandy landings cemented, and questions about the future of the Western alliance itself. For many, the ideals...