The United States entered WWII because of the Japanese attacking pearl harbour. After the Japanese invaded China and started the infamous Rape of Nanking, the US cut off the Japanese oil agreement with them. This only enraged the Japanese even more. The Japanese knew that sooner or later they would need to attack the US, so they tried to do it quickly by attacking Pearl Harbour. Pearl harbour was a US navy base on the island of Oahu, Hawaii. There the US had 50 battleships ready for war. Admiral Tojo of Japan knew that he needed to destry this base in order to win the war. So he launched the surprise attack on December 7th 1941. After doing this President Franklin Roosevelt immediately declared war on Japan and Germany, thus bringing them into WWII.
