Hitler invades the Soviet Union
In June of 1941, less than two years after he had shocked the world by signing a peace treaty with his Communist enemy the Soviet Union, Hitler shocked the world again by sending a huge army to invade his Soviet treaty partners. Having conquered most of Europe, Hitler thought he could get away with anything. In addition, he sincerely hated Communism (and anyone who got in his way).
Hitler was sure his genius planning would have the German army safely in Moscow before winter. During the first weeks of the invasion, the Germans won so much territory and prisoners that it looked like those dreams might come true. The Soviet Union seemed to be on the brink of collapse.
Against the nightmare scenario of the Soviet Union folding like Russia did in World War I, Churchill met with Roosevelt on a battleship off Canada. They weren't supposed to be allies; after all, the U.S. was still officially neutral and not at war. The two took the unusual step of drafting the Atlantic Charter (1941). The Charter said that all people had the right to choose their own government, especially to reinstate the democratic governments that dictators had taken away. It also called for disarmament and peace overseen by an international organization.
When they had time to catch their breath, the leaders of the Soviet Union signed on to the Charter later in 1941. The United States was in the interesting position of dictating war aims for a conflict it was not fighting. The charade got even thinner when America started to convoy supplies as far as Iceland through the German submarine packs with a shoot-to-kill order against attacking U-boats.
Japan attacks Pearl Harbor
In the Pacific, the U.S. also managed to twist the imperial tail of Japan without actually attacking. In late 1940, the United States cut off the shipment of scrap iron and other industrial supplies to Japan; in mid-1941, America froze Japan's investments in the U.S. and cut off all gas and military supplies in response to Japan's move into Indochina. The Japanese war machine was going to grind to a halt without either buying supplies from the U.S. or stealing them from the lightly guarded Dutch West Indies.
America said it would turn on the supplies again if Japan backed out of China, but for Japan that would be a loss of both honor and hard-won territory. Japan pretended to negotiate and got ready for a surprise attack against the United States.
Early on the morning of Sunday, December 7, 1941, with most of the U.S. Pacific Naval Fleet rocking gently at anchor in the tropical breezes of Pearl Harbor, Hawaii, Japanese carrier planes flew in low to drop bombs that sank or disabled almost the whole fleet and killed 2,500 Americans. Unfortunately for the Japanese, they missed sinking the most valuable ships in the Pacific navy, three aircraft carriers that weren't in the harbor at the time. In months to come, these ships would come looking for the Japanese. Given that the U.S. had had no experience at being invaded for the last 125 years, the attack on Pearl Harbor came as a shocking surprise. Within days, the United States was at war with Japan, Germany, and Italy.
Having trouble remembering who's on what side? The major players in World War II were JIG versus SUB. The Axis Powers were Japan, Italy, and Germany against the Allies, who were the Soviet Union, the United States, and Britain. This mnemonic may be simple, but it's also useful.