Although the United States did not join the fight until December 1941, events were underway years earlier that would draw the country into World War II.
The primary event for the United States' entry into the conflict was the Japanese attack on the US Navy at Pearl Harbor. For some time, Japan had been growing concerned over US opposition to its imperialistic advances in Asia and the Pacific. As Japan conquered more of China, the United States responded by threatening to cut off important trade that Japan relied upon. Japan was planning on seizing British and Dutch Pacific colonies in order to gain control over important natural resources, and Japanese command understood that the United States would oppose such a move. That is why the Japanese launched a preemptive strike against the US Navy in the hope that it would debilitate the nation's ability to stop them. This proved to be a miscalculation. Rather than destroy the country's will to fight, Americans were galvanized to wage a total war against the Japanese.
The Lend-Lease Act also led to US involvement in the war. Beginning in March 1941, the United States began lending China, the United Kingdom, and the Soviet Union food, petroleum, and wartime material and equipment. It was understood that these loans were unlikely to be repaid. This was the United States' way of aiding the enemies of the Axis without becoming directly involved. This move angered the Germans and Japanese, who saw the United States as increasingly hostile to their interests.
When the Japanese attacked Pearl Harbor on December 7, 1941, Germany and Italy also declared war on the United States. This was done in solidarity with their Japanese ally and drew the United States into the war in Europe as well as in the Pacific.
Despite its isolationist policies during the 1930s, many Americans were very concerned with the spread of fascism in Europe. Therefore, when Germany and Italy made themselves official enemies of the United States, Americans were quick to join the war effort to defeat them.