Autumn 1937, a former country estate at Butovo, south of Moscow. Trucks come at night from the city's prisons. The condemned are checked against lists, led to long trenches and shot. In fourteen months more than 20,000 people die here, workers, priests, engineers, old Bolsheviks, at one killing site among many.
Why then
The murder of Sergei Kirov, the Leningrad party chief, in 1934 gave Stalin a pretext. Show trials in 1936 and 1937 have old revolutionaries confess to plotting with Hitler and Trotsky. Stalin talks of a 'fifth column' that would betray the country in war. In July 1937 the politburo sends each region a quota of people to shoot or imprison.
What they wanted
Stalin means to remove anyone who might turn against him when war comes. His police chief, Nikolai Yezhov, sets out to beat every quota, and regional bosses ask for higher ones. Ordinary people denounce neighbours to save themselves, or to get the flat.
What changed
Three of the army's five marshals are dead, along with thousands of officers, four years before Hitler invades. Yezhov is arrested in 1939 and shot in 1940. The killing stops when Stalin stops it.
Still argued
The archives show Stalin signing execution lists himself. Historians still argue how far local bosses drove the machine past his orders, and how much was fear of war and how much paranoia.