Perry de Havilland reminds me in the comments of the Katyn Forest massacre. Never mind the Russians’ role in World War II being under-appreciated and not well-known enough. How often does this tale get told? Stalin and Beria ordered the murder of between 16 and 27 thousand Polish officers, who were being held as POWs, as part of their plan to subdue the Poles. All were taken to a forest in Russia (near Smolensk), shot in the back of the head, and thrown into mass graves, which then had the soil put back on, with saplings planted on top to disguise it.
Things got a tad awkward later when the Soviets joined the Allies and the Poles asked if the imprisoned officers could rejoin the fight. ‘Er, we late them all go. Or they escaped,’ the Soviets said. ‘Dunno where they are now.’ Then when the Germans uncovered one of the mass graves in 1943, the Soviets tried to put the blame onto the Germans.