What made the United States join World War II?

America officially only enter World War 2 two years into the war on December 8, 1941, when all but one member of Congress passed the motion one day after the attack on Pearl Habor in Hawaii. Although America formally maintained neutrality up until that moment, the U.S. has long been involved in the war, providing support to the Allies.
In the first two years of the war, America remained politically neutral, but president Franklin was working hard to prepare the Americans for what he regarded to be an inevitable conflict. He felt that the war was threatening U.S. security and tried to find ways to help the European Allies without being formally involved in the war. He persuaded Congress in November 1939 to repeal the arms embargos that were part of the neutrality law and pass the Fourth Neutrality Act, which allowed him to trade arms with countries whose defence he would seem to be vital to the security of the United States. The U.S. would also provide its air force and navy to “escort” British convoys that transported supplies “leased” from America to protect them from enemy submarines. The U.S. military was also deployed to replace British forces in Iceland after the British invasion there.

As had already happened during World War I, the United States tried to remain neutral during the initial European escalations. Even when Germany started its policy of expansion, becoming increasingly fascist, the United Stated did not intervene.
However, the United States was eventually forced into the conflict. On December 7th, 1941, Japan attacked the military base of Pearl Harbor, Hawaii.
The day after the attack, President Roosevelt asked Congress to declare war on the Empire of Japan, therefore officially becoming part of World War II. Today, the reason why Japan attacked Pearl Harbor still remains a mystery, as well as a fact of debate among historians. Many think that the attack was fueled by Japan's aim to expand across the Pacific.
Once the United States entered the conflict, there was a huge shift. They supplied a huge amount of weapons and men and essentially changed the fate of the conflict.
Roosevelt died in 1945 and wasn't able to see the final victory. His successor, Truman, ordered the atomic bombing of Hiroshima and Nagasaki, which definitely ended Japan's resistance as well as the conflict. To this date, the atomic bomb represents one of the greatest tragedies in history. It resulted in an enormous amount of instant deaths, as well as many others due to radiation in the following years.