Following World War I, the United States attempted to return to a state of isolationism. A series of treaties and agreements were entered into, including the famous Washington Disarmament Conference, intended to prevent the horrors of World War I from happening again. Obviously, it did not happen.
United States entry into the war was largely the result of Japanese aggression in the Pacific. Japan had hoped to gain German possessions in the Pacific Basin at the Treaty of Versailles, but came away empty handed. This did not settle well with the Japanese government. The U.S. was at first a passive participant in the European war by furnishing ships, etc. to the British; but did not actively engage in combat. When Japan began an aggressive campaign in the Pacific, and President Roosevelt stopped oil shipments and scrap metal shipments to Japan, the Japanese High Command determined that war with the United States was inevitable. The Japanese plan was to take out the Pacific Fleet and allow time for a negotiated peace before the Atlantic Fleet could be re-directed. They failed to understand American resolve when attacked, best expressed by President James K. Polk at the beginning of the Mexican War: "American blood has been shed on American soil."
The United States declared War on Japan on Monday after Pearl Harbor was attacked on Sunday. Germany and Italy declared war on the United States on Thursday in keeping with the Tri-Partite Pact with Japan.
There was only one event that led the US to enter WWII. That was the Japanese attack on Pearl Harbor in what was then the Territory of Hawaii. That happened on December 7, 1941. No other event actually caused the US to participate in the war.
There were many events that made the US more likely to want to enter the war. Most of those events have to do with German and Japanese expansion. As Germany and Japan attempted to take more territory, the US became more and more worried. Some examples of this include the German invasion of Poland in September of 1939 and the Japanese taking of Vietnam in September of 1940.
However, none of these events made the US enter the war. The US did not enter the war when the German blitzkrieg took most of Western Europe. It did not enter the war when Germany started to bomb England. These events made more Americans sympathize with the Allies and oppose the Axis, but only the attack on Pearl Harbor actually led the US to enter WWII.