Commemorations in the capital are to center around Warsaw's Monument to the Fallen and Murdered in the East, where at 11 a.m. Poland’s President, Andrzej Duda, was scheduled to lay a memorial wreath. Other events were planned nationwide, particularly in the east of the country.

At dawn on September 17, 1939, the Red Army stormed Poland from the east under a secret agreement with Hitler’s Third Reich.

The Molotov–Ribbentrop Pact, officially the Treaty of Non-Aggression between Germany and the Union of Soviet Socialist Republics and named after the two countries’ foreign ministers, contained a secret protocol outlining Berlin’s and Moscow’s spheres of influence across northern Europe.

As a result of the invasion, just 16 days after German forces attacked from the west, Poland was caught between two major military powers. The invasion resulted in over a quarter of a million Polish soldiers being taken prisoner and 1.5 million civilians being deported to the USSR, mostly to Siberia and Kazakhstan.

The country’s eastern borderlands were irretrievably lost. The Soviet Union annexed all the Polish territory under its military control and the Soviet security service, the NKVD, held show elections that led to 13.5 million Poles being made Soviet citizens.

A period of brutal repression ensued, including political murders, the most notorious of which was the execution of over 20,000 Polish officers and intelligentsia in and around Katyn, western Russia, in 1940.

The Soviet occupation of Eastern Poland lasted until the summer of 1941 when Germany terminated its pact with the USSR and invaded its former ally under Operation Barbarossa. The areas taken by Germany were later recovered by the USSR and fell under their control as part of the Yalta Agreement signed in 1945 by Stalin, Churchill and Roosevelt.