When did Hitler declare war on Europe?

To my knowledge he never did. It was war by default when he invaded Poland on the 1st of September 1939. The British declared war on Germany on the 3rd of September 1939.

Thereafter the German Propaganda machine always maintained that Britain had started the war by its declaration. This back-fired badly some time into the war when the Germans spoke of the war as having started on the 1st of September 1939.