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. Instead, three days after December 7th, Italy and Germany declared war on the United States.