Hitler and Stalin's war in the east was a defining episode of the Second World War, a clash of the tyrants in which more than 30 million people died.
BBC2's War of the Century, halfway through its four-week run, tells the story of the fear, savagery and sheer inhumanity that permeated both sides.
The archive footage is often shocking, but what brings the series most chillingly to life are the first-hand testimonies of the perpetrators and victims of atrocity.
This could be your grandad, sitting comfortably in his armchair, talking about how he shot a terrified young deserter "without thinking twice". Or granny, explaining the joy she felt when she executed a captured German soldier.
The opening up of the former Soviet Union has allowed many of these people to talk openly for the first time. So we hear of the murders, rapes and mass persecution that characterised German occupation and tarnished the Russian image of heroic struggle. Speaking of the detachments set up to shoot Red Army deserters, one Russian sums up the ethos that drove - and still drives - people to act beyond the bounds of humanity. "If it was right or wrong - so what? It was war."
War of the Century Tuesdays BBC2 9pm