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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:

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."