Explanation: During WW2, the Soviets and Nazis were effectively aligned for the first two years of the war, carving up and genociding Poland together and dividing Eastern Europe into spheres of influence. Both sides intended to backstab one another, but Hitler acted first, overrunning unprepared and reorganizing Soviet units from eastern Poland all the way into Ukraine in a matter of months under Operation Barbarossa in 1941. Hitler was convinced that the whole Soviet system was rotten (true) and that it would easily crumble when faced with an existential threat (false), saying that only the 'door' needed to be kicked in to destroy the entire country.
Unfortunately for Hitler, but fortunately for all the Slavic peoples he intended on exterminating, the Soviet Union was not that fragile, and when faced with the prospect of mass extermination at the hands of a country which openly regarded them as subhuman, the Soviet people rallied and, at enormous cost, pushed the Nazis back into Germany over the course of four more years of war, coordinating with the Western Allies in the process.