It was a very narrow stretch of land (in some places only 40 km), which divided post–World War I Germany into two parts, East Prussia and Germany proper, and which was part of the old Polish province Royal Prussia. Thus, this arrangement gave Poland free access to the Baltic Sea, which was one of the guarantees proposed by Woodrow Wilson in his Fourteen Points. In addition, the population was predominantly Polish.
Before the Polish Corridor
Historically, the conflict over the region is rooted in the fact, that German exclave East Prussia was located between Poland and Lithuania and had no land connection to Germany, while the Baltendeutsche settled down there.
Around 1000 the southern coast of the Baltic Sea was populated mainly by Slavic peoples, before German settlement began.
In 1772 if one travelled eastward along the southern coast of the Baltic sea, starting from the eastern border of Western Pomerania (in hands of Prussia and the part of Holy Roman Empire), further east was was Polish Pomerania and Polish Warmia, then again Ducal Prussia in hands of Prussia, Lithuania as a part of Poland, Polish Livonia, Courland, Russian Livonia and Estonia. Thus Ducal Prussia was Prussian exclave at the Eastern coast of the Baltic sea.
After the partitions of Poland, Prussia became the ruler of the entire southern Baltic coast to the city of Memel. With the foundation of the German Empire in 1871, Pomerania and East Prussia became parts of Germany, over the protests of Polish deputies in the Prussian and German parliaments.
In 1920, only 30% of the 2.5 million East Prussians used the Polish language, in contrast to Polish Pomerania where out of 1,876,000 people only 190,000 were ethnic Germans.
Treaty of Versailles tried to resolve the question of German-Polish border in the region. Possible solutions would have been to let Germany keep the whole of Prussia (then Poland would continue to be a landlocked country) or to give both West and East Prussia to Poland. Both solutions seemed bad, so a compromise was chosen, that gave Poland access to the sea, adhering to the Fourteen Points of Wilson.
Resolution
In 1933 the Nazi Party took control over Germany. In 1939 Adolf Hitler demanded free, ex-territorial passage of German trains and troops to East Prussia and back. The Polish government refused, and this situation led to the outbreak of the Second World War.
After World War II, at the Potsdam conference the northern part of East Prussia was renamed Kaliningrad Oblast and annexed by Soviet Union, while the southern part Warmia i Mazury fell to Poland. The ethnic German population fled or were forcefully expelled by the occupying Soviets. There was no longer a German exclave in the region. However, after the fall of Soviet Union, Kaliningrad Oblast became an exclave of Russia.