One can easily point to the Japanese attack on Pearl Harbor as being the reason why the United States entered World War II. But while this is certainly true, the answer regarding why the country entered the war is more nuanced than simply thanks to the occurrences of a single event.
Even as late as 1940, U.S. President Theodore Roosevelt had told the American people that their "boys are not going to be sent into any foreign wars." However, British Prime Minister Winston Churchill was persistent in asking for U.S. assistance and participation in World War II. Some believe that Roosevelt was taking steps toward bringing the United States into the war even before the bombing at Pearl Harbor.
After the attack, only one congressman opposed the declaration of war. If the attack hadn't been enough to provoke the United States, then Germany's declaration of war against the United States just four days later certainly would have been.