Polish–Soviet border agreement of August 1945

The Border Agreement between Poland and the USSR of 16 August 1945 established the borders between the Union of Soviet Socialist Republics (USSR) and the Republic of Poland. It was signed by the Provisional Government of National Unity (Tymczasowy Rząd Jedności Narodowej) formed by the Polish communists. According to the treaty, Poland officially accepted the ceding its pre-war Eastern territory to the USSR (Kresy) which was decided earlier in Yalta already. Some of the territory along the Curzon line, established by Stalin during the course of the war, was returned to Poland. The treaty also recognised the division of the former German East Prussia and ultimately approved the finalised delimitation line between the Soviet Union and Poland: from the Baltic sea, to the border tripoint with Czechoslovakia in the Carpathians.

Prior to the First World War, within the Russian Empire Polish territories were administered by a Vistula Land, whose eastern frontier roughly determined the ethnic border between the Polish people on the west, and the Ukrainians and Belarusians (the referred to as Little and White Russians respectively) on the east. In Austrian Galicia there was no administrative border which marked the ethnic one between the Polish and the Galician Ukrainians (Ruthenians).

...
Wikipedia