After the end of World War II in Europe , the Soviet Union signed Sign_agreement a Polish–Soviet border agreement with the new , internationally recognized Know Polish Provisional Government of National Unity on 16 August 1945 .

The Polish–Soviet War Hostile_encounter ( 14 February 1919 – 18 March 1921 ) was fought Hostile_encounter by the Second Polish Republic , the Ukrainian People 's Republic and the proto-Soviet Union ( Soviet Russia and Soviet Ukraine ) over a region comparable to today 's westernmost Ukraine and parts of modern Belarus .