Generally people believe that World War II started in year 1939. Minor German radio station was attacked by the group of Nazis and Hitler to prove their invasion of Poland right. Hitler gave the justification for the invasion that Gleiwitz transmitter was attacked by the regular polish troops.
Britain and France were with Poland so with the invasion of Poland by Germany, Britain and France had to declare war against Germany on September 3 which leaded to World War II.
Other view about the starting of World War II was that the Manchuria was invaded by Japan in 1931. In fact for a long time Japan was trying to invade china and Korea and had planned to takeover whole of the Asia. U.S was totally against this movement so Admiral Yamamoto ordered and attack on Pearl Harbor. Their attack was just 30 minutes before the war was declared which lead to World War II.
It is also commonly believed that the World War II started because many issues were not yet resolved in world war I. U.S did not take any interest in European matters. Germany came to Hitler when it was near economic collapse. So to save Germany and avenge its defeat it invaded Poland which gave a start to World War II.
Another view which has come up as the reason for world war II was the Italyâ€™s invasion of Ethiopia in year 1935 in order to build an empire.
So in short we can say that the main causes behind World War II were the invasion of Poland by Germany and secondly Japanâ€™s attack on pearl harbor and British and Dutch colonies.