In the three previous years, three neutrality acts were passed to keep the States out of foreign wars. After World War I many people considered themselves pacifists. But when faced with totalitarian fascist aggressors, war seemed necessary. Mussolini in Italy and Hitler in Germany wanted to conquer Europe and establish a master race.
Between 1939 and 1945 more than six million Jews were killed in concentration camps in order to "cleanse" Europe of non-Aryan population. This was genocide.
In 1940, France fell to the Germans and the U.S. started sending soldiers to Britain. Roosevelt said he was willing to run for a third term election due to the war crisis. No one had ever served more than two terms as president, but Roosevelt was reelected in 1940. It was this year that Japan joined with Italy and Germany in the Axis powers.
In December 1941 Japan bombed Pearl Harbor. Almost immediately the United States declared war against the Axis Powers formalizing its involvement in World War II.
Paranoia swept the nation, and over a hundred thousand Japanese-Americans were forced to abandon their property and live in detention camps. They were feared to be a security risk. Comparable measures were followed in Canada. This inhumane and embarrassing episode was a major violation of civil rights. Similar actions were not taken on German-Americans because they could not be so immediately recognized by physical appearance.
In order to end the war abruptly with fewer casualties, the atom bomb was dropped on Hiroshima on August 6th. The Japanese Prime Minister had been warned to surrender, but he had refused. Over 70,000 Japanese were killed. A second bomb was dropped on Nagasaki three days later, and Japan surrendered.
Anticipating the need for a new alliance to prevent future world wars, arrangements for the United Nations were made in 1942. The 24 countries fighting the Axis Powers agreed to sign the Declaration of the United Nations. They all promised not to make separated peace agreements with the enemy. Everyone was in this together. A Security Council was established. The seats in the Council rotated, and only the US, China, France, Great Britain, and the USSR held permanent places. The UN had no power to act if any of these five powers vetoed the resolution. A major goal of the United Nations was to prevent the use of the atom bomb again.