Final answer:
The United States entered World War II after the attack on Pearl Harbor by the Japanese air force on December 7, 1941. Prior to this, the U.S. had stayed neutral despite Japan's territorial expansions. The attack led the U.S. to declare war on Japan, marking its official involvement in the conflict.
Step-by-step explanation:
The United States entered World War II following the attack on Pearl Harbor by the Japanese air force on December 7, 1941. This surprise attack led to the destruction of the American naval fleet stationed at Pearl Harbor, Hawaii. Prior to this, despite Japan's territorial expansions, America had chosen to stay out of the conflict. In response to this attack, the United States declared war on Japan, marking its involvement in the war.
In August 1941, the United States had imposed sanctions on Japan, including an embargo on oil and gas sales, to pressure them into halting their aggression. This embargo, however, reinforced Japan's plan to obtain the natural resources they needed from the South Pacific and the Philippines, a U.S. colony. Consequently, Japan, viewing the U.S. as an enemy unwilling to make sacrifices for war, planned the attack on Pearl Harbor.
Following America's declaration of war, Germany and Italy also declared war against the U.S., leading to the U.S.'s involvement in the European theater of the war. The attack on Pearl Harbor also resulted in China joining the Allies, although it did not participate in the fighting in Europe.
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