Why was the war that started in 1939 called World War II?

The war that
started in September of 1939 was calledeven before it had truly become a world war.  It was
first called that in Time Magazine of September 11, 1939.  At that point, the war only included
Germany, which had invaded Poland, and Britain and France, who had declared war when Germany did
that but had yet to take any real action.

The reason that Time called it WWII
was because the previous great war had come to be known as WWI and it seemed likely that this
war would end up involving many more countries than were involved right at the beginning.  There
were so many points of potential conflict around the world that it seemed certain the war would
become much bigger than it was.