This was not the first time Mr. Biden has made off-the-cuff remarks about foreign policy during an evening fund-raiser. At another such event in New York in October, he warned donors that the world was closer to nuclear destruction than at any time since John F. Kennedy faced off against Nikita S. Khrushchev over Soviet missiles in Cuba in 1962. “We have not faced the prospect of Armageddon since Kennedy and the Cuban Missile Crisis,” Mr. Biden said then, prompting a nervous reaction. In California on Tuesday, Mr. Biden was…