1967 was the Summer of Love in the US, with hippies trying a bit of 'shroom-tinged communism in the trendier parts of San Francisco.
In 1968, the Czechs tried for a bit less communism and a bit more Western-style freedom, only without quite so many drugs or non-bathing hippies and more fun consumerism.
The Soviets were not amused.
On the night of August 20th, the tanks of five Warsaw Pact nations rumbled across the Czech border, shot up a few people, parked a toady of Moscow in the driver's seat, and shut down the borders (but only when they realized that about 70,000 people had split.)
So remember, kids: All you need is love!
And an RPG.