The evidence strongly supports attributing the outbreak of World War II in Europe to Hitler. As leader of Nazi Germany, he ordered the invasion of Poland on September 1, 1939, which triggered the declarations of war that conventionally mark the war’s start. The main caveat is that this is historical shorthand for a broader, multi-causal conflict.
Caveats
The wording compresses broader causes of the war, including earlier diplomatic failures, appeasement, and Nazi expansionism.
Some historians distinguish the 1939 European outbreak from the wider Asia-Pacific conflict, which had major antecedents before 1939.
The invasion was carried out by Nazi Germany as a state, but the evidence indicates Hitler was the decisive authorizing leader.