What event led to the United States entering World War II in 1941?

The United States declared war on Japan on December 8, 1941. The declaration was a direct response to the Japanese attack the previous day on the U.S. naval base at Pearl Harbor on Oahu island, Hawaii. President Franklin Delano Roosevelt spoke a joint session of Congress, and his speech was broadcasted to the entire nation via radio. In an address that has become legendary, President Roosevelt referred to December 7 as “a date which will live in infamy.”
Because the United States had a substantial presence in the Pacific, Japan had already been worried that it would enter the war. The Japanese intention was to inflict as much damage as possible on the portions of the U.S. fleet that were still in port. The Japanese attack forces and supporting air and seagoing vessels included more than four hundred airplanes and six aircraft carriers. The initial attack was carried out by about 180 airplanes. It began around 8 A.M. on a Sunday morning, which was calculated to be a time that the U.S. forces would not be expecting an attack. A second wave quickly followed the first.
Because it was a surprise attack, damage was swift and very serious. The U.S.S. Arizona immediately sustained tremendous damage, and then exploded and finally sank. On that ship alone, more than a thousand service members were killed. Despite the valiant American defense, more than 2,400 Americans were killed and almost 1,200 more were wounded. From both the Navy and the Air Corps, 169 airplanes were destroyed. However, no U.S. aircraft carriers were then docked, there so none was destroyed.
Once President Roosevelt had been notified (it was lunchtime in Washington), he called together the Secretaries of War and Defense, other cabinet members, and military advisers. He soon requested that Congress prepare an official declaration of war, which was accomplished by the following morning.
https://www.britannica.com/event/Pearl-Harbor-attack

https://www.history.com/news/pearl-harbor-facts-wwii-attack

https://www.ourdocuments.gov/doc.php?flash=old&doc=73