Final answer:
World War II began due to Adolf Hitler's expansionist policies and desire for a German empire, the rise of fascism, and the Japanese invasion of China. The United States joined the war following Japan's attack on Pearl Harbor, amidst a backdrop of global reactionary shifts due to the failures of capitalism and democracy during the Great Depression.
Step-by-step explanation:
Causes of World War II
The causes for the outbreak of World War II are multi-faceted. A significant factor was Adolf Hitler's ambition to create a German empire, which led to aggression and expansionist policies by Nazi Germany. Furthermore, the rise of fascism in Italy and the Japanese invasion of China in 1937 played crucial roles in escalating tensions. The international community's reaction to the Great Depression, largely seen as a failure of capitalism and democracy, contributed to the rise of totalitarian regimes that offered alternative socio-political models. The situation worsened with the military actions of the Axis powers, which included Germany, Italy, and Japan.
The United States entered World War II primarily due to the Japanese attack on Pearl Harbor on December 7, 1941. However, the stage was set much earlier with the global crises and conflicts of the 1930s. President Franklin Roosevelt had been seeking ways to assist in the fight against German advances in Europe even before the direct attack on the United States. Additionally, long-standing issues between Japan and the United States pertaining to power dynamics in the Pacific also contributed to the eventual confrontation.