Like were Germans just told it was their Aryan right to expand their borders as much as possible? Or did the Nazi's frame imperial conquest of neighboring territories some other way?
At August 31st 1939 the SS made a false flag operation against the German radio station in Gleiwitz (Gliwice), dressed in polish uniforms.
Next day (September 1st) Germany started the invasion of Poland. Hitler justified the operation with the faked Polish attacks on German territory:
This night for the first time Polish regular soldiers fired on our territory. Since 5.45 A.M. we have been returning the fire, and from now on bombs will be met by bombs. Whoever fight with poison gas will be fought with poison gas. Whoever departs from the rules of humane warfare can only expect that we shall do the same. I will continue this struggle, no matter against whom, until the safety of the Reich and its rights are secured.
(Address by Adolf Hitler before the Reichstag, September 1, 1939)