<P> German--Polish relations has been marked by an extensive and complicated history . </P> <P> From the 10th century onward, the Piast Kingdom of Poland established under Duke Mieszko I had close and chequered relations with the Holy Roman Empire . However, these relations were overshadowed by the centuries - long Polish--Teutonic Wars, as a result of which the Teutonic Duchy of Prussia became a fief of the Kingdom of Poland . Prussia retained a certain level of autonomy under Polish rule . Later, the Kingdom of Prussia rose and eventually became one of the partitioners of Poland in 1772--1795 . </P> <P> In 1918, Poland regained its place on the map . After World War I, the Treaty of Versailles deprived Germany of its territories in West Prussia, East Upper Silesia and Danzig (Gdansk) and transferred them all to Poland . It was seen as a great injustice in the Weimar Republic, in part leading to the Nazi takeover of power in 1933 . On 1 September 1939, Poland was invaded by Germany, thus initiating World War II . The Third Reich established concentration camps in German - occupied Poland, the biggest located in Auschwitz . Poland suffered many casualties and huge damages during the war . After World War II, Germany lost its former eastern territories to Poland and the Soviet Union . In 1945 - 1950, a series of expulsions and fleeings happened, in which up to 16 million ethnic Germans were forced to leave their homes in Poland and resettle in post-war Germany . It was the largest forced movement of any population in history . </P> <P> The Cold War saw good relations between the communist states of People's Republic of Poland and the German Democratic Republic . Polish - West German relations, on the other hand, remained bad, although they improved after Chancellor Willy Brandt launched the Ostpolitik . In 1990, Germany reunified and it confirmed the Polish - German border on the Oder - Neisse line in a treaty . Both states are now European Union allies and partners, having an open border and being members of the European Single Market . The once bad relationship between Poland and Germany has now become a strategic partnership . </P>

What was the issue between germany and poland that was the cause of world war ii
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