Causes of WWII
German U boats in the Atlantic: German submarines patrolled much of the Northern Atlantic, sinking allied ships. Played a significant part in the Battle of the Atlantic (fought throughout the war. Huge force used in Economic warfare/blockades. Main problem for naval fighting in the Atlantic ocean.
Cash and carry policy: Developed to keep Germans from getting supplies from the U.S cause they couldn't pay cash up front or transport the supplies back to their bases. Cash and carry policy was all about not helping the Germans without blatantly saying "nope, we can't do that." Germans didn't take well to this but it allowed us to weaken Germans slightly while strengthening France and Great Britain. This was the U.S basically picking their allies.
Lend-Lease Act of 1941: The act authorized the president to transfer arms or any other defense materials for which Congress appropriated money to "the government of any country whose defense the President deems vital to the defense of the United States." Britain, the Soviet Union, China, Brazil, and many other countries got materials under this law. Germany did not benefit from this act at all. Hitler hated that we had twice kept him from getting materials.
Bombing of Pearl Harbor: The attack on Pearl Harbor was a surprise military strike by the Imperial Japanese Navy against the United States naval base at Pearl Harbor, in the United States Territory of Hawaii, on the morning of December 7, 1941. This was the final cause for the U.S to join the war officially. When the U.S joined the war they started to turn the tide by backing Japan up and away from the islands. Following shortly would be the fall of Hitler and the end of WWII.