In What caused the US to enter world war ii?

America was helping Britain on the quiet, using lease lend of ships in return for America being allowed to gain bases on British controlled countries. It was the Japanese attack on Pearl Harbour that brought America to declare war on Japan. Then Germany, as an ally of Japan, also declared war on America. This brought America directly into becoming a member of the Allied forces and to help fight against Germany and Japan. It was the industrial might and vast manpower of America that swung the tide against Nazis Germany and Japan's aim of conquering large areas of the Earth.