On 17 Sep 1939, Hitler’s de facto ally, the Union of Soviet Socialist Republics better known as the Soviet Union, invaded Poland from the east as the Poles were fighting the Germans coming from the west.
By 9 September 1939, Polish mobilization was complete the Poles and were holding their own along the Vistula and in the Carpathians against the German attack. They even launched a large counterattack at Bzura and repulsed the initial German attacks on Warsaw. Unfortunately on 9 Sep the German propaganda minister Josef Goebbels announced to the world that the Germans had reached Warsaw. The German people thought they had won and were jubilant. Goebbels ran with it. Poland had no way of contradicting Goebbel’s message. The British, French, and Soviets all soon believed Poland was lost. The mistaken belief absolved the Brits and French from any further assistance, and on the 11th, Stalin decided he’d better invade Poland before the Germans took it all.
On 17 September 1939, eight days after the Poles were supposedly defeated by the Germans, Soviet forces crossed the Polish frontier from the east, and made defense along the Vistula pointless. Initially Polish units on the eastern frontier thought that the Soviets were coming to Poland’s assistance, but that notion was quickly dispelled. On 25 Sep, the Polish government announced the evacuation of the country. The last Polish army unit only surrendered on 6 Oct – a month after the war had supposedly been lost.
In occupied Eastern Poland the NKVD, the forerunner of the KGB, immediately arrested and summarily executed tens of thousands of Polish army officers and NCOs, politicians, police officers, business owners, priests, school teachers, and university professors, anyone exhibiting leadership qualities. The Red Army sacked, tortured, raped, and killed its way through eastern Poland in a prelude of what would happen to Germany 5 1/2 years later. Hundreds of thousands Poles were sent to slave labor camps in Siberia. Sham elections were held by the NKVD to give an air of legitimacy to the brutal occupation. Anyone who ran against their preferred candidate was killed, and anyone who voted against them was sent to Siberia.
“The liberation of Poland (by National Socialist Germany and Communist Soviet Union) is an example of cooperation of socialist nations against Anglo-French imperialism.” – The Communist International, 7 Oct 1939