<P> On December 7, 1941, the Empire of Japan launched an attack on the US naval and army base on Pearl Harbor in Hawaii, beginning a war between Japan and the United States . Japan had not informed its ally, Germany, in advance of the attack, although the Japanese ambassador had informed the German Foreign Minister, Joachim von Ribbentrop, at the beginning of December that relations between the US and the Japanese Empire were at a breaking point, and that war was imminent . He was instructed to ask Germany for commitment to declare war under the terms of the Tripartite Pact should that occur . Hitler and Ribbentrop had been urging Japan to attack and take over Singapore from the British, on the theory that doing so would not only hurt the UK, but would also serve to help keep the US out of the war . </P> <P> According to the terms of their agreements, Germany was obliged to come to the aid of Japan if a third country attacked Japan, but not if Japan attacked a third country . Ribbentrop reminded Hitler of this, and pointed out that to declare war against the US would add to the number of enemies Germany was fighting against, but Hitler dismissed this concern as not being important, and, almost entirely without consultation, chose to declare war against the US, wanting to do so before, he thought, Roosevelt would declare war on Germany . </P> <P> In fact, Hitler's declaration of war came as a great relief to British Prime Minister Winston Churchill, who feared the possibility of two parallel but disconnected wars--the UK and Soviet Union versus Germany in Europe, and the US and the British Empire versus Japan in the Far East and the Pacific . With Nazi Germany's declaration against the United States in effect, American assistance for Britain in both theaters of war as a full ally was assured . It also simplified matters for the American government, as John Kenneth Galbraith recalled: </P> <P> When Pearl Harbor happened, we (Roosevelt's advisors) were desperate...We were all in agony . The mood of the American people was obvious--they were determined that the Japanese had to be punished . We could have been forced to concentrate all our efforts on the Pacific, unable from then on to give more than purely peripheral help to Britain . It was truly astounding when Hitler declared war on us three days later . I cannot tell you our feelings of triumph . It was a totally irrational thing for him to do, and I think it saved Europe ." </P>

When did the united states declare war on germany in world war ii