Prague Spring involved the Warsaw Pact invasion of.

Czechoslovakia

Explanation:

The invasion of Warsaw Pact member states (except Romania) into Czechoslovakia began at 11 am on the evening of August 20 of 1968 with the crossing of the Warsaw Pact countries' combined forces across the Czechoslovak borders.

After half past three in the morning on August 21, airplanes, sirens and engines were heard in Prague. At three, all the lights were off in the capital. Speakers from the radio said that former allies were treacherous, that aggression had been committed, that the attack on Prague was a crime unknown in international law. At about three in the morning, Soviet commandos occupied the Prague airport, and then tanks flew from large planes toward the city center.