What factors influenced the US decision to enter WWII?

The US decision to enter WWII was primarily influenced by the Japanese attack on Pearl Harbor and global political dynamics.

The attack on Pearl Harbor on December 7, 1941, was a significant turning point that directly led to the US's involvement in WWII. Prior to this event, the US had adopted a policy of neutrality, largely due to the isolationist sentiment prevalent among the American public following the devastation of World War I. However, the surprise military strike by the Japanese Navy against the US naval base at Pearl Harbor in Hawaii resulted in significant American casualties and damage to the Pacific Fleet. This attack outraged the American public and shifted public opinion in favour of war, leading President Franklin D. Roosevelt to ask Congress for a declaration of war against Japan the following day.

In addition to the attack on Pearl Harbor, global political dynamics also played a crucial role in the US's decision to enter the war. The aggressive expansionist policies of Nazi Germany and Imperial Japan posed a direct threat to international peace and stability. The US had been providing material support to the Allies (including Britain, China, and the Soviet Union) through the Lend-Lease Act, reflecting its concern over the Axis Powers' actions. The US's entry into the war can thus be seen as a response to the global threat posed by the Axis Powers.

Furthermore, economic factors cannot be overlooked. The US economy had been severely affected by the Great Depression, and war production for the Allies had already begun to stimulate economic growth. Full-scale involvement in the war further boosted the economy, creating jobs and pulling the US out of the Depression.

Lastly, ideological factors also influenced the US's decision. The democratic US was ideologically opposed to the fascist regimes in Germany and Italy and the militaristic regime in Japan. The US's entry into the war can thus be seen as a defence of democratic values against totalitarian ideologies.