How did World War 2 Start?

What do you think started World War 2? Some people think that it was started on September 1st 1939 when Hitler and the Nazis faked a Polish attack on a German radio station so that the Nazis could declare war on Poland. An hour later, Hitler declared war on Poland. Britain and France had a pact with Poland, thus they declared war on Germany on September 3. Others say that in 1931, Japan invaded Manchuria. The Japanese invasion of China in 1937 was also considered.