17 September 2026 marks 87 years since Soviet troops entered eastern Poland. The attack left the Polish state fighting on two fronts in the opening weeks of the Second World War.

Nazi Germany had invaded Poland on 1 September 1939. Britain and France declared war on Germany on 3 September. On 17 September the Soviet Union attacked from the east. Poland, already engaged against the Wehrmacht, now also had to defend its eastern border.

The United States Holocaust Memorial Museum places the decision in the context of the German-Soviet Non-Aggression Pact of August 1939 and its secret protocol. The protocol divided spheres of influence in Eastern Europe. The 17 September invasion put the Soviet side of that arrangement into effect on the ground.

The Polish government left the country the same day. Warsaw continued to resist until 28 September. The last operational Polish unit surrendered on 6 October. Nazi Germany and the Soviet Union subsequently divided Polish territory according to the arrangements between the two powers.

The occupations that followed had grave consequences for civilians, including Jews in Poland, who were already targeted by Nazi Germany’s policy of persecution. 17 September 1939 does not close the Polish campaign; it changes it, adding a second occupying power.

The archive photograph used here shows a German-Soviet parade at Brest-Litovsk on 22 September 1939, several days after the eastern attack, not the border crossing of 17 September. Eighty-seven years later, the date remains essential to understanding the opening of the war and Poland’s position between the two powers.

Photo: Bundesarchiv, Bild 101I-121-0012-30. German-Soviet parade at Brest-Litovsk, 22 September 1939 (not the 17 September crossing). CC BY-SA 3.0 DE. Cropped and upscaled to 16:9.

Source consulted: Invasion of Poland, Fall 1939 | USHMM; German-Soviet Pact | USHMM.