On December 7, 1941, the Japanese navy attacked the American fleet at Pearl Harbor, thus bringing America into WWII. The Japanese army was facing a shortage of fuel and material as America embargoed airplane fuel and scrap iron to Japan because of its aggressive role in China and Southeast Asia. Japanese leadership thought the best course of action to take would be to attack the American fleet at Pearl Harbor, crippling any American offensive action against Japan and potentially even convincing America not to get into the war. What the Japanese did not realize was that it made most Americans pro-war out of a desire to avenge the deaths of over 2000 American servicemen. While the Japanese damaged or sunk the majority of the cruisers and battleships stationed at Pearl Harbor, they did not damage the vital oil supplies on the island or the aircraft carriers, which were out on maneuvers that morning and not located in the harbor. America quickly replaced the battleships, and the aircraft carriers proved key in early conflicts with Japan such as Doolittle's bombing raid over Tokyo and the Battle of Midway, both of which happened in 1942 and played decisive factors in turning the tide of the war.
When America declared war on Japan, Germany also declared war on the United States; by the end of 1941, all the key players of WWII were in place.