If Hitler had been left alone, he probably wouldn't have had any reason to attack France.
One of the untold stories of World War II was that the United States actually "liberated" France from the Nazis. The reality is that a substantial portion of the French population was perfectly content to live under the influence of the Third Reich -- particularly in light of the strong Marxist influence in France in the 1930s.
The Blum government in France, in fact, was cobbled together in 1936 under the Socialist, Communist, and Radical Socialist parties -- and during much of the next ten years France was essentially in a state of a quiet civil war. Leon Blum himself was imprisoned by the Vichy government for the duration of World War II.
I disagree, so we'll have to leave it at that. Basically, Britain and France were bound by treaty to defend Poland if it were attacked. They did nothing in its defense, but Hitler had to know that invading Poland would trigger a declaration of war by France and Britain.