Prague Spring - Wikipedia

On August 20, 1968, during the night 200,00 Warsaw Pact Soviet led troops begin to invade Czechoslovakia in response to the Prague Spring. From the article:

"As these talks proved unsatisfactory, the Soviets began to consider a military alternative. The Soviet Union's policy of compelling the socialist governments of its satellite states to subordinate their national interests to those of the "Eastern Bloc" (through military force if needed) became known as the Brezhnev Doctrine.[46] On the night of 20–21 August 1968, Eastern Bloc armies from four Warsaw Pact countries – the Soviet Union, Bulgaria, Poland and Hungary—invaded the ČSSR.[47][48]

That night, 200,000 troops and 2,000 tanks entered the country.[49] They first occupied the Ruzyně International Airport, where air deployment of more troops was arranged. The Czechoslovak forces were confined to their barracks, which were surrounded until the threat of a counter-attack was assuaged. By the morning of 21 August Czechoslovakia was occupied.[48]

Neither Romania nor Albania took part in the invasion.[50] Soviet command refrained from drawing upon East German troops for fear of reviving memories of the Nazi invasion in 1938.[51] During the invasion by the Warsaw Pact armies, 72 Czechs and Slovaks were killed (19 of those in Slovakia), 266 severely wounded and another 436 slightly injured.[52][53] Alexander Dubček called upon his people not to resist.[53] Nevertheless, there was scattered resistance in the streets. Road signs in towns were removed or painted over—except for those indicating the way to Moscow.[54] Many small villages renamed themselves "Dubcek" or "Svoboda"; thus, without navigational equipment, the invaders were often confused.[55]

Although, on the night of the invasion the Czechoslovak Presidium declared that Warsaw Pact troops had crossed the border without the knowledge of the ČSSR government, the Soviet Press printed an unsigned request – allegedly by Czechoslovak party and state leaders – for "immediate assistance, including assistance with armed forces".[56] At the 14th KSČ Party Congress (conducted secretly, immediately following the intervention), it was emphasized that no member of the leadership had invited the intervention.[57] More recent evidence suggests that conservative KSČ members (including Biľak, Švestka, Kolder, Indra, and Kapek) did send a request for intervention to the Soviets.[58] The invasion was followed by a previously unseen wave of emigration, which was stopped shortly thereafter. An estimated 70,000 fled immediately with an eventual total of some 300,000.[59]

The Soviets attributed the invasion to the "Brezhnev Doctrine" which stated that the U.S.S.R. had the right to intervene whenever a country in the Eastern Bloc appeared to be making a shift towards capitalism.[60] There is still some uncertainty, however, as to what provocation, if any, occurred to make the Warsaw Pact armies invade. The days leading up to the invasion was a rather calm period without any major events taking place in Czechoslovakia.[29]"