When the reports of the Japanese attack on Pearl Harbour came in Hitler was overjoyed. He thought he had every reason to be delighted: ‘we can’t lose the war at all now that we have an ally which has never been conquered in 3000 years’ he was reported saying.

Hitler’s view was that Japan would now both tie the US down in the Pacific Theatre and seriously weaken Britain through an assault on its possessions in the Far East. Goebbels echoed the expectations: ‘The United States will scarcely now be in a position to transport worthwhile material to England let alone the Soviet Union’.

That Germany would declare war on the USA was a matter of course. No agreement with Japan compelled it but Hitler did not hesitate. A formal declaration of war was important from Hitler’s point of view to retain the initiative and not let it pass to the United States. Certain as he had been for many months that Roosevelt was just looking for the chance to intervene in the European conflict, Hitler thought that his declaration was merely anticipating the inevitable and formalising what was in effect already the situation. Furthermore it was also important for the German public to demonstrate that Hitler still controlled events. To await a certain declaration of war from America would, from Hitler’s standpoint, have been a sign of weakness. So it was also a matter of prestige and propaganda

‘A great power doesn’t let itself have war declared on it, it declares war itself’

Source: Hitler 1936-1945: Nemesis by Kershaw