Today in history: On March 31, 1968, at the conclusion of a nationally broadcast address on Vietnam, President Lyndon B. Johnson stunned listeners by declaring, “I shall not seek, and I will not accept, the nomination of my party for another term as your president.” Also on this date: In 1492, King Ferdinand and Queen Isabella of Spain issued the Alhambra Decree, an edict expelling Jews from Spanish soil, except those willing to convert to Christianity. In 1854, Japan and the United States