The Cultural Revolution
Red Guards, struggle sessions, and a state turned against its own institutions.
What happened
From 1966 Mao mobilised the young against the Party apparatus itself, calling on students — the Red Guards — to "bombard the headquarters" and destroy the "Four Olds" (old ideas, culture, customs, habits). Schools and universities closed for years. Officials, teachers and "class enemies" were paraded in dunce caps, beaten and driven to suicide in mass "struggle sessions"; neighbours and children were pressed to denounce one another. When the Red Guards' factional violence spun out of control, Mao had the army rein them in and sent some 16 million urban youth "down to the countryside."
The results
Death estimates run from roughly several hundred thousand to two million (Dikötter puts the toll at 1.5–2 million; others lower), with vastly more careers, educations and families wrecked than lives taken. A decade of a nation's development was consumed by a convulsion ordered from the top and aimed at its own society — ending only with Mao's death in 1976 and the arrest of the "Gang of Four."
Why it happened
This was a dictator weaponising a youth movement to dismantle the very institutions — the Party bureaucracy, the courts, the press — that might one day have checked him. It was a power struggle dressed as a revolution. And it ended when the man who launched it died, because in a system with no independent veto, nothing short of that could stop it.