Flight of Poles from the USSR
The flight and forced displacement of Poles from all territories east of the Second Polish Republic (Kresy) pertains to the dramatic decrease of Polish presence on the territory of the post-war Soviet Union in the first half of the 20th century. The greatest migrations took place in waves between the Bolshevik Revolution of 1917 and in the aftermath of World War II in Europe.
Source: Wikipedia — Flight of Poles from the USSR (CC BY-SA 4.0)