"[I]f the Kaiser in World War I and the Fuehrer in World War II had been on notice that an armed attack against any of the friendly nations with whom we associate ourselves would be considered a cause even for us to consider and study and determine whether or not we would enter into the common defense, it would have stopped both those wars before they occurred, and in my opinion that one single designation of a commonalty of interest in the North Atlantic pact... is the best assurance against World War III."