Polish Corridor, strip of German territory awarded to newly independent Poland by the Treaty of Versailles in 1919. The strip, 20 to 70 mi (32–112 km) wide, gave Poland access to the Baltic Sea. It contained the lower course of the Vistula, except the area constituting the Free City of Danzig (see Gdańsk) and the towns of Toruń, Grudziąz, and Bydogoszcz. Gdynia was developed as Poland's chief port and came to rival the port of Danzig. Free German transit was permitted across the corridor, which separated East Prussia from the rest of Germany. Although the territory had once formed part of Polish Pomerania, a large minority of the population was German-speaking. The arrangement caused chronic friction between Poland and Germany. In Mar., 1939, Germany demanded the cession of Danzig and the creation of an extraterritorial German corridor across the Polish Corridor. Poland rejected these demands and obtained a French and British guarantee against aggression. On Sept. 1, 1939, the Polish-German crisis culminated in the German invasion of Poland and World War II.