The Treaty of Versailles made the port of Danzig a free city under League of Nations control so that Poland would have access to the sea. Danzig had been the cause of some debate at the conference as it was heavily populated by Germans and ceding it to Poland or making it a free city meant East Prussia would be cut off from the rest of Germany. But Poland needed access to the sea, and it was hoped by giving it the status of a “free city”, it would ease the pain for the German population a little. However, though a “free city”, Danzig would have a customs union with Poland and Poland would be responsible for the city’s foreign affairs. And Germany also lost a significant strip of land surrounding the city: the ‘Polish Corridor’. This was significant not least because it was the “corridor” that cut East Prussia off from the rest of Germany.
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Danzig was a particular sore point with Hitler. And it was with Danzig that Hitler began his demands with respect to Poland. He wanted it returned to Germany and he wanted control of the transport links in the Polish Corridor.
In his early demands he said that he was prepared to accept an independent Poland (of course, he wasn’t) so long as it was closely tied to Germany. In October, 1938 the Germans tried to get Poland to agree to return Danzig and allow Germany to build a motorway and a railway across the Polish corridor so as to link Germany with Danzig and East Prussia which would necessitate extraterritorial rights. But the Poles were not having any of it.