When World War 2 began, the United States had entered into a period of isolationism. However, as tensions in Europe and in the Pacific grew, the United States began to slowly edge toward its entrance into the war. Japan had entered into the Tripartite Pact with Germany and Italy. Japan continued its aggressive attacks on islands in the Pacific seeking oil and resources, but eventually, the Japanese realized that they would have to reduce the capabilities of the U.S. Navy in the Pacific, so the plot to attack Pearl Harbor was formed. The morning after the attack in December 1941, the United States declared war on Japan, and true to the Tripartite Pact, Germany and Italy in response, declared war on the United States. The United States had officially entered into World War 2. Its capacity to produce thousands of tanks, airplanes, and ships for itself and its allies proved to be a crucial factor in the Allies’ fight against the Axis powers. Also, the leadership provided by the American military contributed to the eventual Allied victory. There were many factors that caused the United States to enter the war such as, growing tensions between the U.S. and Germany and the bombing of Pearl Harbor, and as a result of its entrance, it provided military power, resources, and new technology.