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German–Japanese Axis-cooperation

Historian Ian Kershaw suggests that this declaration of war against the United States was a serious blunder made by Germany and Italy, as it allowed the United States to join the war in Europe and North Africa without any limitation. On the other hand, American destroyers escorting convoys had been effectively intervening in the Battle of the Atlantic with German and Italian ships and submarines, and the immediate war declaration made the Second Happy Time possible for U-boats. Franklin D. Roosevelt had said in his Fireside Chat on 9 December 1941, 2 days before the European Axis powers formally declared war on America, that Germany and Italy already considered themselves to be in a state of war with the United States. Plans for Rainbow Five had been published by the press early in December 1941, and Hitler could no longer ignore the amount of economic and military aid the US was giving Britain and the USSR.