The Polish–Soviet War was fought primarily between the Second Polish Republic and the Russian Soviet Federative Socialist Republic before it became a union republic in the aftermath of World War I and the Russian Revolution, on territories which were previously held by the Russian Empire and the Habsburg Monarchy following the Partitions of Poland.

Eastern Borderlands or simply Borderlands was a term coined for the eastern part of the Second Polish Republic during the interwar period (1918–1939). Largely agricultural and extensively multi-ethnic with a Polish minority, it amounted to nearly half of the territory of interwar Poland. Historically situated in the eastern Polish–Lithuanian Commonwealth, following the 18th-century foreign partitions it was divided between the Empires of Russia and Austria-Hungary, and ceded to Poland in 1921 after the Treaty of Riga. As a result of the post-World War II border changes, all of the territory was ceded to the USSR, and none of it is in modern Poland.