On this day in military history…

During the night of 20–21 August 1968, one of the largest military operations undertaken in Europe since the end of the Second World War began as armies of the Warsaw Pact crossed the borders of Czechoslovakia. Soviet troops, supported by forces from Poland, Hungary and Bulgaria, moved into the country in an operation intended to crush a political reform movement that Moscow feared could weaken communist control across Eastern Europe. Within hours, tanks were moving through Prague, aircraft were landing at strategically important airfields and thousands of foreign troops were occupying key government buildings, communications centres and military installations.

The invasion brought an abrupt end to the period of political liberalisation that became known as the Prague Spring. It also demonstrated the determination of the Soviet Union to maintain control over the communist states of Eastern Europe, even if that required the use of overwhelming military force against another member of the Warsaw Pact.

The crisis had developed following the appointment of Alexander Dubček as First Secretary of the Communist Party of Czechoslovakia in January 1968. Dubček, a Slovak communist who had spent part of his early life in the Soviet Union, did not initially intend to abandon socialism or withdraw Czechoslovakia from the Warsaw Pact. Instead, he believed that the country's increasingly unpopular and rigid political system required substantial reform.

Czechoslovakia had been under communist control since 1948, and by the 1960s the country was experiencing economic difficulties, political stagnation and growing dissatisfaction among intellectuals, students and sections of the Communist Party itself. Dubček responded by introducing a programme that became described as "socialism with a human face".

Censorship was relaxed, greater freedom of speech and expression was permitted and newspapers began openly discussing subjects that had previously been forbidden. Political prisoners were rehabilitated, criticism of government officials became increasingly common and there was discussion of economic reforms intended to reduce rigid central planning. Many Czechoslovaks began to believe that communism could be transformed into a less authoritarian system while remaining within the socialist bloc.

The Soviet leadership under Leonid Brezhnev viewed events very differently. Moscow feared that the reforms could eventually undermine the Communist Party's monopoly on power. Of even greater concern was the possibility that similar movements might spread into Poland, East Germany, Hungary and other Warsaw Pact countries.

The memory of the Hungarian Revolution of 1956 remained particularly significant. In that uprising, Soviet forces had intervened militarily after the Hungarian government attempted to break away from Moscow's control. Thousands were killed and the revolution was crushed. Twelve years later the Soviet leadership was determined not to allow another communist state to slip from its grasp.

Throughout the spring and summer of 1968, increasing pressure was placed upon Dubček and the Czechoslovak government. Soviet leaders repeatedly demanded that the reforms be restricted and warned of what they described as "counter-revolutionary" forces operating inside the country.

Meetings took place between Czechoslovak and Soviet officials, including important talks at Čierna nad Tisou near the Soviet border at the end of July. Dubček attempted to reassure Moscow that Czechoslovakia remained loyal to the Warsaw Pact and to socialism, but he resisted demands to reverse many of the reforms.

At the beginning of August, leaders from several communist states met in Bratislava. A declaration appeared to suggest that a compromise had been reached, but the Soviet leadership continued preparing for military intervention.

Behind the diplomatic discussions, Warsaw Pact forces had already been conducting large-scale military exercises in and around Czechoslovakia. These manoeuvres gave Soviet commanders valuable knowledge of the country's roads, communications network and military installations. When the political decision to invade was finally taken, enormous forces were already positioned close to the borders.

The operation was given the Soviet codename Operation Danube. During the evening of 20 August 1968, Warsaw Pact units began moving into Czechoslovakia from several directions. The principal invading force was Soviet, but Polish, Hungarian and Bulgarian troops also participated. East German forces were prepared for involvement, although their direct participation inside Czechoslovakia was limited, partly because of the historical sensitivity surrounding the German occupation of the country during the Second World War.

Altogether, the initial invasion involved roughly a quarter of a million troops, although the number of occupying soldiers eventually rose considerably higher. Thousands of tanks, armoured vehicles, artillery pieces and aircraft supported the operation.

The Czechoslovak armed forces numbered several hundred thousand men and possessed substantial quantities of tanks, aircraft and artillery. In purely military terms they might have been able to inflict significant losses upon an invading army. The government, however, ordered the military not to resist.

This decision almost certainly prevented the invasion from developing into a major European war. Czechoslovak commanders were instructed to remain in their barracks and avoid armed confrontation. Most complied, although the invading forces encountered widespread civilian resistance.

One of the most important objectives of the invasion was Prague's Ruzyně Airport. Soviet planners understood that controlling the airport would allow large numbers of airborne troops and reinforcements to be flown directly into the capital.

During the opening stages of the operation, a Soviet civilian aircraft reportedly requested an emergency landing. Once the airport had been secured, Soviet military transport aircraft began arriving in rapid succession carrying airborne troops, vehicles and equipment. These forces moved immediately towards important government buildings, radio stations and other strategic locations.

At the same time, armoured columns advanced towards Prague and other major cities. Residents awoke to the extraordinary sight of Soviet tanks moving through streets that only hours earlier had appeared peaceful.

Many invading soldiers had been told that they were entering Czechoslovakia to defend socialism against a counter-revolution or foreign-backed coup. Some reportedly expected to encounter NATO troops. Instead they were confronted by civilians carrying Czechoslovak flags, shouting slogans and demanding that the invaders return home.

One of the most dramatic centres of resistance was the headquarters of Czechoslovak Radio in Prague. Radio broadcasts had become an important symbol of the Prague Spring, and journalists continued transmitting reports of the invasion while appealing to the population to remain calm and avoid unnecessary bloodshed.

Thousands of civilians gathered around the radio building. Soviet tanks and troops attempted to secure the area and violent confrontations followed. Vehicles were set on fire, barricades appeared in the streets and gunfire was heard. A number of civilians were killed.

Across the country, however, resistance was mainly non-violent. Road signs were removed, painted over or deliberately altered to confuse invading troops. Street names were changed and villagers sometimes gave Soviet soldiers false directions. Almost every road sign seemed to point towards Moscow.

Civilians surrounded tanks and attempted to argue with their crews. Some climbed onto armoured vehicles and waved Czechoslovak flags. Others painted slogans such as "Ivan, go home" on buildings, roads and military vehicles.

In several incidents, Soviet tank crews appeared uncertain and confused by the hostility of the population. Young conscript soldiers sometimes found themselves confronted by people who spoke Russian and demanded to know why they had invaded a supposedly friendly socialist country.

The situation occasionally became deadly. Shots were fired at crowds, military vehicles struck civilians and tanks crushed vehicles and barricades. By the end of 1968, more than one hundred Czechoslovak civilians had been killed as a direct result of the invasion and several hundred more were seriously wounded.

The political leadership was quickly targeted. Alexander Dubček and several other senior reformist politicians were arrested by Soviet forces and transported to Moscow.

Their removal created an extremely dangerous political situation. The Soviet leadership initially hoped that a new government could be installed that would support the intervention. This proved difficult because the invasion was widely condemned inside Czechoslovakia and many Communist Party officials refused to cooperate.

The Czechoslovak population continued supporting Dubček. Even the Communist Party held an emergency congress that condemned the occupation and reaffirmed support for the reform leadership.

Moscow therefore chose to negotiate with the detained Czechoslovak leaders rather than immediately replacing them. Under enormous pressure, Dubček and his colleagues were forced to sign what became known as the Moscow Protocol.

The agreement effectively accepted the presence of Soviet troops and required the Czechoslovak leadership gradually to reverse many of the reforms introduced during the Prague Spring.

Dubček initially returned to office, but his political position had been fatally weakened. During the following months censorship was gradually restored, reformers were removed from important positions and the country's political system was brought back under strict Communist Party control.

In April 1969 Dubček was replaced as First Secretary by Gustáv Husák. Under Husák, Czechoslovakia entered a period officially described as "normalisation". In reality, it meant the systematic dismantling of the Prague Spring.

Thousands of reform-minded politicians, academics, journalists, military officers and civil servants lost their positions. Many were expelled from the Communist Party. Newspapers and broadcasting organisations returned to strict censorship and political dissent was suppressed.

Hundreds of thousands of Czechoslovaks eventually left the country, either immediately after the invasion or during the years that followed. Many settled in Western Europe, North America and Australia.

The invasion also had enormous international consequences. Western governments condemned the Soviet action, although there was little possibility of direct military intervention. Czechoslovakia lay firmly inside the Soviet sphere of influence and any NATO military response risked escalating into a confrontation between nuclear-armed superpowers.

The United States was also heavily involved in the Vietnam War at the time, while Western leaders recognised that the political division of Europe remained largely fixed.

The invasion contributed to what became known as the Brezhnev Doctrine. Under this principle, the Soviet Union claimed the right to intervene in another socialist country if communist rule there appeared to be threatened.

In effect, the sovereignty of Warsaw Pact states was limited whenever their domestic policies conflicted with Soviet interests. The doctrine would influence Soviet foreign policy for many years.

The invasion also damaged the international reputation of communism. Communist parties in Western Europe, including those in Italy and France, criticised the Soviet intervention. Many left-wing intellectuals who had previously sympathised with the Soviet Union became increasingly disillusioned.

Within the Warsaw Pact itself, one country openly refused to participate. Romania, led by Nicolae Ceaușescu, condemned the invasion and called it a violation of national sovereignty. Albania, which had already moved politically away from Moscow, formally withdrew from the Warsaw Pact following the invasion.

One of the most tragic symbols of resistance emerged several months later. On 16 January 1969, 20-year-old student Jan Palach set himself on fire in Wenceslas Square in Prague in protest against the occupation and the growing public resignation that followed it.

Palach died three days later. His funeral became a major demonstration against Soviet control, and he remains one of the most powerful symbols of resistance to the occupation.

Soviet troops did not quickly leave Czechoslovakia. Under an agreement signed in October 1968, a large Soviet military force remained permanently stationed in the country. Tens of thousands of Soviet soldiers would remain there for more than two decades.

The political system created after 1968 survived until the revolutionary changes that swept across Eastern Europe in 1989. In November that year, mass demonstrations began in Prague and other cities. The peaceful uprising became known as the Velvet Revolution.

Within weeks the Communist Party lost its monopoly on power. Václav Havel, the dissident playwright who had spent years opposing the regime, became president of Czechoslovakia.

Alexander Dubček, whose reforms had been crushed twenty-one years earlier, returned to public life and received an emotional welcome from crowds in Prague.

The withdrawal of Soviet forces began following the collapse of communist rule and was completed in 1991.

The invasion of Czechoslovakia remains one of the defining military and political events of the Cold War. It was an operation carried out with enormous military strength but very little genuine popular support. The Warsaw Pact possessed overwhelming numerical superiority, yet it faced a population whose resistance largely relied upon radios, demonstrations, painted slogans and simple acts of defiance rather than weapons.

For the Soviet leadership, Operation Danube achieved its immediate objective. The Prague Spring was destroyed, Czechoslovakia remained inside the Warsaw Pact and Soviet influence over the country was secured for another twenty years.

But the invasion also revealed a fundamental weakness within the Soviet-controlled system. A supposedly allied country had to be occupied by hundreds of thousands of troops simply because its people wanted greater freedom within socialism.

The tanks that entered Prague on 21 August 1968 crushed the reforms of the Prague Spring, but they did not destroy the ideas behind them. Those ideas survived through two decades of censorship and occupation, eventually returning to the streets during the Velvet Revolution of 1989, when Czechoslovakia finally regained the political freedom that had been taken away at gunpoint in August 1968.