Hitler had earlier promised the Japanese that if they attacked the Americans he would delcare war on the U.S. as well. He kept his promise, likely fearing that

Hitler had earlier promised the Japanese that if they attacked the Americans he would delcare war on the U.S. as well. He kept his promise, likely fearing that the Japanese might settle quickly otherwise, or because in many ways, esp at sea, he was already at war with the U.S.