"Danzig war und ist eine deutsche Stadt."
"Danzig was and is a German city."
-- Adolf Hitler's speech to the Reichstag, 1 September 1939
What if Danzig remained as a part of East Prussia after WW1? What if the Polish Corridor was not the sovereign territory of Poland, but rather was a League of Nations-mandated international territory? This would strip Hitler of his casus belli against Poland.
Would Hitler find another excuse to wage war on Poland? What would it be?