mar 1, 1939 - Hitler annexes Czecholslovkia
In March 1939, Hitler disobeyed an agreement made at the Munich Conference and annexed the rest of Czechoslovakia, as well as demanded for territory in Poland. This went against not only the Treaty of Versailles, but also the UN. However, nations like Britain and France allowed it to happen since they were big on appeasement: it is better to give Hitler something then start a war. This gave the US reason to enter the war because they were worried about Hitler gaining too much power if he continued to expand territory wise and military wise.
Added to timeline:
Causes of US Entering WWII-Meaghan and Felicia