To Hitler, the Japanese attack on Pearl Harbor, on 7 December 1941, came as a bolt from the blue, since the Japanese had not informed Berlin of their plans to attack America. He greeted the news with intense joy. He was convinced that the Japanese would keep America occupied in the Pacific, and that would lead to a significant decline in the American supplies to the Soviet Union and Britain.
The Tripartite Pact between Germany, Italy, and Japan did not oblige Germany to aid Japan if it were the aggressor. At that time, the German troops were in retreat in Moscow, but Hitler was filled with hubristic pride. To the acclamations of “Sieg heil,” he said to his party members, “A great power doesn’t let itself have war declared on it—it declares war itself,” and he went on to declare that Germany and Italy were at war with America, alongside Japan. On 11 December 1941, Ribbentrop summoned the American chargé d'affaires in Berlin and read out the text of Germany’s declaration of war on America.
When Churchill heard of the news from Pearl Harbor, he felt relieved. He said that, on the night of 7 December, he “went to bed and slept the sleep of the saved and thankful.” Churchill knew that Britain did not stand a chance against Germany. He had one plan for defeating Germany: drag the Americans into the war. By attacking Pearl Harbor, the Japanese had silenced America’s isolationist lobby and made it easy for Roosevelt to enter the war. Despite the bad news from Malaya and Hong Kong, Churchill now knew that Britain would not be defeated.