Russian aggression has very long history, which modern Russia tries to forget. In the beginning of the Second World War, Hitler and Stalin were equal aggressors who invaded Poland from different sides. According to the Molotov- Ribbentrop Soviet-Nazi agreement, 52% of Poland had to become Soviet territory. Hitler and Stalin have agreed to invade Poland simultaneously from both sides. However, Stalin, who had his own plans of establishing communism in the whole Europe, didn’t keep his promise and invaded Poland only 17 days later, which allowed Soviets to look like “liberators”. Invasion of Poland by Soviet troops in 1939 was explained to Soviet people as protection of brotherly Ukrainian and Belorussian people from the German aggressor. Pretty much in the same way as modern Russian government explains to Russians its current invasion of Ukraine – as protection of brotherly Russian people from Ukrainian “fascists”.
Polish army didn’t have any chance. By 17 September 1939, about two weeks after German invasion of Poland, the Polish defence was already broken and the only hope was to retreat and reorganize in the east. However, on September 17th, 800,000-strong Soviet Red Army hit in the back and invaded Poland in violation of the Riga Peace Treaty, the Soviet-Polish Non-Aggression Pact, and other international treaties, both bilateral and multilateral. In total, 3,000–7,000 Polish soldiers died fighting the Red Army, with 230,000–450,000 taken prisoner. The Soviets also executed all the Polish officers they captured after the Battle of Szack, on 28 September 1939, and over 20,000 Polish military personnel and civilians perished in the Katyn massacre.
This video shows one example of Russia’s shameful history of collaborationism with fascist Germany: the German–Soviet military parade in Brest-Litovsk (https://en.wikipedia.org/
wiki/German%…). The Battle of Brest-Litovsk took place between 14 and 17 September 1939. After three days of heavy fights for the stronghold in the town of Brześć, the Germans captured the fortress and the Poles withdrew. According to the terms of the 1939 German-Soviet Nonaggression Pact the territory around Brest as well as 52% of the then Poland was assigned to the Soviet Union. The Soviet 29th tank brigade under Brigadier Semyon Krivoshein reached the area of Brześć on September 21st and took over the fortress from the Wehrmacht. During that event a joint German-Soviet parade was held in the town, after which the German forces left the area. The XIX motorized Corps of Wehrmacht and the 29th Tank Brigade of the Red Army participated in the parade.