For a few fleeting months in 1968, Czechoslovakians living under Communism got to enjoy newfound freedoms in a period known as the “Prague Spring.” But in August of that year, tanks from the Soviet Union and allied Warsaw Pact nations quickly crushed the reforms.

Just like that, the Iron Curtain—which divided the Soviet Union and its allies in eastern and central Europe from the West during the Cold War—forced Czechoslovakia back under Soviet control. While the suppression of attempted reforms was severe, two decades later, Czechoslovakia finally cast off Kremlin control following the Velvet Revolution.

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The reform era started under new Czechoslovakian leader Alexander Dubcek, who shook up the political establishment by implementing freedom of the press, freedom of speech and freedom of travel, along with economic reforms. These liberalization efforts, which he called “socialism with a human face,” won popular support from his citizens.

Czechoslovakia was a member of the Warsaw Pact, a mutual defense group of nations led by the Soviet Union, and several fellow member states were alarmed by the reforms. The Soviet Union, East Germany, Poland, Hungary and Bulgaria met that summer to decide how to respond.

Dubcek made it clear that he wasn’t looking to pull his nation out of the Warsaw Pact, but that wasn’t good enough for the Soviets.

“He misunderstood the deal with the Soviet Union—he believed if he remained loyal on foreign policy, that they could push the envelope at home,” says Simon Miles, a Cold War historian and professor at Duke University’s Sanford School of Public Policy. “Where in fact, because of the Soviet leaders’ own anxieties about their position, what mattered more was what you did at home, and you had a little bit more leeway on foreign policy.”

Several hundred thousand Soviet, Polish, Hungarian and Bulgarian troops invaded Czechoslovakia on August 20. Miles says that East Germany was pulled out of the invasion at the last minute, “because it is perceived in Moscow that in 1968, the image of Germans invading Czechoslovakia is going to be bad,” referring to Nazi Germany’s invasion of Czechoslovakia in 1939.

There was little in the way of armed resistance to the Warsaw Pact invasion, but protesters flooded the streets, some confronting tanks with flowers, taking down street signs to confuse the soldiers, and yelling “Ivan, go home.”

Warsaw Pact troops shot to death more than 100 protesters. The Soviet Union, which implausibly claimed it had come in at the invitation of the Czechoslovakia government, snuffed out the reform experiment. The invading army arrested Dubcek and flew him to Moscow. He returned to Prague on August 27, fighting back tears as he addressed his nation.

“We hope that you will trust us even though we might be forced to take some temporary measures that limit democracy and freedom of opinion,” Dubcek said.

The Soviet-led invasion provoked condemnation from not just the United States and its Western allies, but also other Communist nations such as China, Yugoslavia and Romania. But U.S. President Lyndon Johnson didn’t take any significant action beyond canceling a summit meeting with Soviet leader Leonid Brezhnev.

That muted response was driven by several factors, including fear of provoking a nuclear war, the goal of getting Soviet help with peace talks in Vietnam, and continuing arms control negotiations with the Soviet Union.

Referring to arms talks with the Soviets, White House press secretary George Christian said, “I don’t know of any change in the president’s very earnest desire to move along any contacts that might be fruitful in that particular area.”

“Underlying the administration’s reaction,” the New York Times reported, “was its view that the East-West line of military confrontation and ideological influence across Europe had long been established, and that neither Washington nor Moscow could move across it without risking World War III.”

The 1968 invasion took place just six years after the Cuban Missile Crisis, which was still fresh in the minds of U.S. officials. And the Johnson administration was hamstrung by the fact that Czechoslovakia, while technically an independent nation, was behind the Iron Curtain and within the Soviet Union's sphere of influence.

On top of that, Johnson had his hands full with the Vietnam War, which he was desperately trying to wind down before his term ended as president in a few months. LBJ had announced earlier in the year he wouldn’t seek re-election.

The invasion took place in the midst of a heated U.S. presidential campaign, just days before the tumultuous Democratic National Convention in Chicago. Political analysts viewed the Soviet aggression as a boon to Republican presidential candidate Richard Nixon, a longtime anti-Communist hawk.

But while Nixon condemned the invasion and said that people who value freedom should demand the troops’ removal, he didn’t call for any forceful U.S. response. And key sentences from his statement strikingly echoed Johnson’s. For example, Johnson said that the invasion “shocks the conscience of the world,” while Nixon called it “an outrage against the conscience of the world.”

There was a reason the longtime national figures sounded like they were reading from the same songbook. The night before, LBJ had called Nixon to brief him on the attack, and remind him that “politics stops at the water’s edge,” a transcript of the call shows. Nixon assured the president that “I won’t say a damn word that’s going to embarrass you, you can be sure of that.”

It was left to Ronald Reagan, who had lost the GOP nomination to Nixon, to call for a “trade and communications quarantine” of the Soviet Union, in a foreshadowing of the confrontational approach he would take as president.

In 1989—two decades after Dubcek’s attempt to reform communism from within— then premier of the Soviet Union, Mikhail Gorbachev, called the 1968 invasion of Czechoslovakia wrong, stating in an article published in the Soviet newspaper Pravda, that the USSR should not have interfered with the nation's attempt at reforms.

By the late 1980s, anti-Soviet movements spread across its satellite nations, including the Velvet Revolution in Czechoslovakia. In 1989, Dubcek gave his first public speech since he had been driven from power, at a rally in Bratislava.

The Communist leadership resigned a few days later, and the following month, dissident and writer, Václav Havel, was elected president. Dubcek was elected speaker of the Parliament.