Final answer:
The direct cause of World War II was Adolf Hitler's aggressive expansionist policies for Germany. The German-led Axis powers' aggression directly started the war, with the U.S. entering after Japan's attack on Pearl Harbor in 1941.
Step-by-step explanation:
The most direct cause of World War II was neither slavery in the United States, German genocide of Jews, nor the assassination of Archduke Ferdinand, which actually triggered World War I. Instead, the primary cause of WWII was Adolf Hitler's aggressive expansionist policies aimed at creating a vast German empire. These aggressive actions led by Germany, along with its Axis allies Italy and Japan, directly instigated the start of the war. It's important to note that the German aggression, fascist expansionism, and the broader context of the Great Depression paving the way for authoritarian regimes across Europe and Asia contributed significantly to the conditions that sparked the global conflict. Moreover, the United States' entry into the war in 1941 was a direct result of Japan's attack on Pearl Harbor, which signified the joining of global powers into an already raging European conflict.