Many reasons, including that Nazi Germany was sinking American supply ships because the US was providing financial and military support to the Allied troops (England, France, China and Russia).
Most importantly, Japan attacked the US naval base at Pearl Harbor, Hawaii, without a declaration of war or any warning that hostlities were being commenced, sinking most of the battle fleet.
The day after the attack, December 7th, 1941, President Roosevelt went before the US Congress and asked for a formal declaration of War with Japan in retaliation. With the exception of Janet Rankin (Montana), the US Congress voted unamiously for War.
President Roosevelt never asked for war with Italy or Germany before the Congress. …show more content…
It starts with the Japanese invasion of Manchuria, China. The Japanese invade Manchuria on September 18th, 1931. China was in the middle of a depression (much like our own) so the Japanese quickly get a strong footing in Southern Manchuria. At this time, the United States is good friends with the Russians. It starts pissing off Russia that the Japanese are taking over China. If it pisses off the Russians, it pisses off us. Also, the Japanese invading China violated the Kellogg-Briand Pact of 1928 which renounced war as national policy. So, the United states tells Japan if they don't get out of China, we'll stop trading oil with them. Japan stays in China, so, naturally, we stop trading oil. Eventually, Japan is forced out of China, but not without a lasting grudge against us Americans. Of course they hate us after this, so when Pearl Harbor was left for an entire day with everyone on holiday, they bombed the Hell out of it. America was looking for a very good reason that everyone could agree on to join the war. FDR had been wanting to join the war all along, and when the Japanese bombed us 10 years later, it had everyone in such an uproar that no one would back down from the