Why did the United States enter World War II?

Why this is the answer

Pearl Harbor is the heart of this answer. On December 7, 1941, Japan attacked the U.S. naval base at Pearl Harbor on the island of Oahu, Hawaii. The National Archives calls it one of the greatest military surprises in the history of warfare. In less than two hours the Pacific Fleet was devastated, and more than 3,500 Americans were killed or wounded. The two countries had not even broken off diplomatic relations. The next day President Franklin Roosevelt went before a joint session of Congress and obtained a declaration of war. The Senate voted for it unanimously, only one member of the House voted no, and Roosevelt signed the declaration that afternoon. Japan's partners, Germany and Italy, then declared war on the United States, and the country was in a true world war, with battles in Europe, Africa, Asia and the Pacific. That is why USCIS accepts four answers that look different but tell one story. Two give the cause: "(Bombing of) Pearl Harbor" and "Japanese attacked Pearl Harbor." Two give the sides: the United States went in to support the Allied Powers, listed as England, France and Russia, and to oppose the Axis Powers, Germany, Italy and Japan. The Axis trio is the same one the older 2008 test asked about directly, when it asked who the United States fought in World War II. The war itself had started two years earlier, in 1939, when Germany invaded Poland. The United States stayed out until it was attacked. It then fought until 1945. Japan signed its formal surrender aboard the USS Missouri in Tokyo Bay on September 2. The easy mix-up runs both ways with World War I. German attacks on ships brought the country into the first war; the Japanese attack on Hawaii brought it into the second. If you hear "World War II," think of December 7 and the attack on a U.S. naval base, and the right answer will follow.