Your question is not specific but I will give you some insight on World War II that may help you answer this question:
There 2 events that are most mentioned which triggered World War II. The first would be the Marco Polo Bridge Incident between China and Japan on July 7, 1937.
Another historical even that has been mentioned to have started WWII was invasion of Germany in Poland in September 1, 1939. Because of this France and Britain declared war on the Nazi's.