Delimitation of the eastern state boundary between the Republic of Poland and the Union of Soviet Socialist Republics (USSR) after the World War II was a resultant of many factors: USSR's act of aggression against Poland in September 1939, the 3 Axis Powers' diplomatic endeavors and conference during the World War II as well as imposing Soviet solutions on Poland resulting from the presence of the Red Army within the territory of Poland and the seizure of power by the communistic left wing block, which was realizing J. Stalin's political goals at the cost of staying in power. It was in Moscow's interest to retain territorial annexations acquired in 1939, which was expressly proved by the confidential agreement signed on 27th July 1944 between 'Polski Komitet Wyzwolenia Narodowego' (The Polish Committee of National Liberation) and USSR, according to which the Polish-Soviet border was to go along the 'Curzon line'. Despite signing a border agreement, the Soviet authorities continued to treat the Bialystok Area as a part of western Belarus districts, which implied that the future of this area was not completely settled. It was only after recall in March 1945 of the group headed by 'Comrade Tsanava', People's Commissar for State Security of USSR, when the Polish status of this area was finally settled. This decision was further confirmed by signing in Moscow on 16th August 1945 an agreement on the course of the eastern border, this time a formal one. Under this agreement the Polish-Soviet Mixed Delimitation Commission was established, which together with Sub-Commissions started to demarcate the border, among the others, in the sector of the Bialystok Province. The course of a new border entailed numerous problems, mainly connected with repatriation. of the Belarus population who, according to the agreement of 9th September 1944 was subject to repatriation provisions, but was not too willing to a free-will change of residence, and was induced to do so by officials' mandatory decisions. A demarcated Polish-Soviet border after the World War II resulted not only in a loss of 1/3 of the territory by Poland but also led to severance of historical and economic ties between 'new Poland' and its former lands - Eastern Borderlands. A demarcation of the Polish-Soviet border after the World War II was made under the rule of international law with the approval of western countries: Great Britain and the United States of America. All declarations that had been promoted during the war were forgotten, i.e., mainly the Atlantic Charter of 14th August 1941, under which the parties to the Chart declared to respect territorial integrity of the countries and any changes thereof were to be subject to the interested parties' consent, as well as the establishments of the Yalta Conference of February 1945, according to which the adopted 'Curzon line' was to be moved in Poland's favor by 5 - 8 km. Unfortunately, it has not happened. Opportunism and state interest prevailed over the credibility of western politicians' attitudes, who approved those changes.