On 21 August 1968, Soviet tanks ended the so-called 'Prague Spring' when they rolled into the Czechoslovakian capital and reasserted Moscow's power.
The world was shocked by the sheer duplicity of the USSR's attack - mere weeks before, the reformist leader Alexander Dub?ek had met with the Soviet leadership and agreed that his reforms were not a threat.
The invasion provoked outrage in the West. Dub?ek and his allies were arrested and all the changes wrought by his regime were undone until the fall of the Berlin Wall more than two decades later.
Watch original British Pathe footage from the Soviet invasion, above.