What were the causes of U.S. entry into World War II?
The United States entered the war because of the Japanese bombing of Pearl Harbor. The United States had a vested interest in the war in Europe; and had supplied ships to Britain through the Lend Lease Program as the British Navy was America's primary protection from attack in the Atlantic. The U.S. did not become involved in the war in Europe until Germany and Italy declared war on the U.S. three days after the U.S. declared war on Japan.
The U.S. had become increasingly alarmed at Japanese aggression in the Pacific, but did not plan to attack. In fact, even when an attack from Japan seemed imminent, it was considered best to allow the Japanese to strike the first blow. President Roosevelt had previously suspended shipments of scrap metal and oil to Japan, which severely crippled its war effort. The Japanese military command saw war as inevitable, and on the advice of Adm. Isuroku Yamamoto (who personally opposed the war) bombed Pearl Harbor with the intent of taking out the Pacific Fleet. The Japanese believed the U.S. did not have the stomach for a long war, and would negotiate a peace settlement before the Atlantic Fleet could steam to the Pacific. They were wrong. They had committed the cardinal sin against America, first expressed by President James K. Polk in the Mexican War of 1848: "American blood has been shed on American soil."
The most immediate reason for the US's actual entry into WWII was the Japanese attack on the US naval base at Pearl Harbor in what was then the Territory of Hawaii. The Japanese attack led to the US declaring war on Japan. From there, Germany declared war on the US and the US was fully involved.
The Japanese attacked Pearl Harbor because they wanted to gain an empire in Asia. They felt that they needed an empire so as to have their own supply of resources like iron and oil. They felt the US would try to stop them from gaining this empire and so they attacked Pearl Harbor to try to disable the US fleet and prevent it from resisting as they took their Asian empire.
Before that, Pres. Roosevelt had been getting the US more involved in the war in Europe, but only by giving aid to the Allies. He felt that this was important because he felt that Hitler was a major menace to US security.
So, the US started getting involved in the war because FDR thought Hitler was a danger to the US. But the US only actually entered the war because Japan attacked Pearl Harbor.