With what country's action did World War II start on September 1, 1939?
On September 1, 1939, when Germany invaded Poland, it is remembered as the date when World War II started.
Without a formal declaration of war,Germany invaded Poland. Britain and France declared war on Germany on September 3, 1939 and all the members of the Commonwealth of Nations, except Ireland, rapidly followed suit.
This war, World War II, was a global war that lasted from 1939 to May 1945.