Comment by shimman

Declaring war is one thing, but if you look at how leaders actually responded it's another (notice the 8 month gap from the declaration of war, into actual fighting). They were still willing to negotiate with Hitler, because most western leadership also wanted the communists to be destroyed and thought Hitler would do just that without attacking them. They were willing to push for this literally until the tanks were invading their streets.

Once Hitler invaded France the "phoney war" turned into a real war. [1]

https://en.wikipedia.org/wiki/Phoney_War