USSR "Liberation of the fraternal peoples of Western Ukraine and Western Belorussia" - 1940

USSR issued on 4 April 1940 a set of five stamps to commemorate the "Liberation of the fraternal peoples of Western Ukraine and Western Belorussia" (parts of Poland at the time) which occurred on 17 September 1939.

The set included various denominations and designs:  10, 30, 50, and 60 kopecks, as well as 1 ruble. 500,000 sets were printed. 

These stamps were part of a series intended to legitimise the Soviet annexation of the eastern territories of the Second Polish Republic, an action that was carried out in accordance with a secret clause of the Molotov–Ribbentrop Pact with Nazi Germany. The Soviet narrative presented the move as a protection and reunification of Ukrainian and Belarusian populations. 

The Soviets quickly began confiscating, nationalizing, and redistributing all private and state-owned Polish property. During the two years following the annexation, the Soviets arrested approximately 100,000 Polish citizens across Kresy.  

In August 2009, on the occasion of the 70th anniversary of the Soviet invasion, the authoritative Polish Institute of National Remembrance announced that its researchers reduced the estimate of the number of people deported to Siberia to 320,000 in total. Some 150,000 Polish citizens perished under the Soviet rule.