What was the event that led to World War 2?

World War Two was a global event, and actually was a combination of several wars. That being said, the accepted start of WWII was Hitler's invasion of Poland in 1939, as it led to the declarations of war by most of the Allied Powers.

In the Far East, the Second Sino-Japanese War would merge into the greater conflict. The Japanese, at some level or another, had been at war for most of the 20th Century prior to the "start" of WWII. The start of the Pacific War could be said to have begun with the occupation of Korea, or the Mukden Incident in 1931, or even the Attack on Pearl Harbor in 1941, which brought the U.S. into both parts of the war.