The most significant factor that directly led to the US entering World War II was the attack on Pearl Harbor by Japan on December 7, 1941. The surprise assault on the US naval base in Hawaii resulted in the loss of thousands of American lives and significant damage to the Pacific Fleet. This act of aggression prompted an overwhelming public outcry and a declaration of war by the United States against Japan the following day.
Unrestricted Submarine Warfare:
Prior to the attack on Pearl Harbor, tensions had been escalating between the US and Japan due to Japan's aggressive expansion in the Pacific region. One key issue was Japan's use of unrestricted submarine warfare against merchant ships, which threatened American interests and supply lines. The sinking of several US vessels and the loss of American lives increased public pressure for action against Japan.
Economic Sanctions and Embargo:
The US had imposed economic sanctions and an oil embargo on Japan in response to its aggressive actions in Asia. These measures aimed to weaken Japan's military capabilities and deter further expansion. However, the embargo also had the unintended consequence of pushing Japan towards more desperate actions, ultimately contributing to the decision to attack Pearl Harbor.
Support for Allies:
The US maintained close ties and provided significant support to its European and Pacific allies who were already at war with Germany and Japan. The fall of France and the threat posed by Nazi Germany in Europe, as well as the Japanese advances in the Pacific, compelled the US to take a more active role in the global conflict.
Isolationist Sentiment:
Despite these factors, there was a strong isolationist sentiment within the US prior to World War II. Many Americans were reluctant to become involved in foreign conflicts and believed that the country should focus on its own internal affairs. However, the events leading up to December 1941, including the attack on Pearl Harbor, eroded public support for isolationism and swayed public opinion in favor of intervention in the war.