I have been looking into this lately, technically it was the United States that declared war on Japan and by association Germany, but Germany formally declared war on the United States a few days later. Germany had been fighting Great Britain and the United States had been feeding them supplies to keep them from total collapse. There was no declaration of war by the US against Germany, but there had been a growing number of conflicts that resulted in sunken ships or U boats on either side.

From what I can tell, Hitler did not take the United States as seriously as he should have, because they kept both Great Britain AND Soviet Russia in the war single handedly, without the US aid, its likely the war would have been a lot different. Partnering with Japan...I dont know I guess it seemed like a good idea at the time. I just feel this is what turned the US against Germany, when it may have been fighting Soviet Russia instead. I am no armchair general. I thought I understood the war fairly well till recently. But to be fair, the United States only emerged as a major power midway through the war, so I guess. Since Germany had no oil of its own, it was forced to take it from countries that had it, mainly Russia. With the US feeding Russia supplies, it became a losing game for Germany to conquer the country in the required time.