One reason relations grew worse in 1949 to 1955 was because the Cold War spread to Asia in the Chinese Revolution, which was soon followed by the Korean War in 1950. The Soviets equipped North Korea with weaponry, while the Americans, who believed Communism was trying to take over the whole world, committed thousands of troops to the conflict. This was not the most important reason for growing tensions, however, because the Korean War ended in stalemate in 1953 and Asia remained relatively quiet for the next few years.
More important than the Cold War in Asia in worsening relations was the creation of military alliances. In response to the Berlin Blockade a military alliance of Western Powers, NATO, was established in 1949 to challenge the Red Army. Soviet fears were then emphasised when West Germany joined NATO in 1955, prompting the creation of a Communist military alliance, the Warsaw Pact. There were now two opposing armed camps deeply suspicious of each other and planning for the possibility of World War Three.
Above all, it was the nuclear arms race that preoccupied the leaders on both sides and made the military alliances so dangerous. In 1949 the Russians exploded an A-bomb, thereby ending American complacency about its nuclear monopoly. The stakes were raised higher when a new generation of weapons, the much more powerful H-Bomb, was developed by the USA in 1952. The Americans were particularly alarmed because it took the Russians only ten months to catch up and invent their own H-Bomb. The reason why the nuclear arms race is the most important factor is because the superpowers could threaten each other with complete and utter destruction
One reason Khrushchev invaded Hungary was because he knew the Red Army, numbering millions of soldiers, had overwhelming power. The Hungarian army was no match and the Americans, for all their talk about 'rolling back the iron curtain', were unlikely to intervene. President Eisenhower was facing re-election on a 'peace ticket' and he was distracted by the Suez Crisis. Moreover, the Soviet possession of a nuclear arsenal was a major deterrent.
The Soviet Union's military capability, however, does not explain the root cause of Khrushchev's decision to invade. He was very concerned that the reform programme of Nagy, the new Hungarian prime minister, was a challenge to communist principles. The prospect of free elections, abolishing censorship and developing trade with the west was too radical for Khrushchev to ignore because these measures went beyond his willingness to accept 'different roads to socialism'.
The most important aspect of Nagy's policies that triggered Khrushchev into taking action was the decision for Hungary to quit the Warsaw Pact. This was completely intolerable because it threatened Soviet authority not only in Hungary but among all of the satellite states of Eastern Europe. Khrushchev could not afford to set a precedent and, furthermore, his own personal position as General Secretary would be untenable in the eyes of his Kremlin colleagues if he didn't invade.
One reason the Berlin Wall was built was because Khrushchev believed President Kennedy was weak. Khrushchev met Kennedy at the Vienna Summit in 1961 and was not impressed, concluding that the new President was young, inexperienced and a rich playboy who would not resist his plans for a wall preventing movement between the two halves of the city. This was not a vital factor, however, because he would probably have constructed the wall regardless who was President since the alternative candidate in the 1960 election, Richard Nixon, did not advocate a tougher line than JFK.
A more important reason than the personality of the American President was the wider context of the end of the thaw. Tension had already been mounting between the superpowers ever since Khrushchev invaded Hungary in 1956, and this escalated during the U2 crisis in 1960 in which an American spy plane was shot down over Soviet air space. Khrushchev walked out of the Paris Summit in protest, showing his increasing aggressiveness and reluctance to negotiate with the Americans. A crisis over Berlin was therefore likely to occur.
The end of the thaw, however, does not explain the root cause of problems in Berlin. The most important reason for building the wall was to stop the flow of refugees. During the 1950s almost 3 million people had used Berlin as a gap in the iron curtain to escape to the West. Khrushchev was determined to stop this exodus because they were mostly young and skilled, the people East Germany could least afford to lose. Moreover, the refugees, who were seeking a better life under capitalism, undermined communist propaganda claims about the benefits of the 'Workers Paradise'. This was key because Khrushchev had to take radical action or risk losing credibility.
One reason why the relationship changed for the worse was because in 1968 Brezhnev ordered Warsaw Pact forces to crush the 'Prague Spring', an attempt by Dubcek to reform Czechoslovakia. The Americans protested because they had welcomed Dubcek's effort to introduce democratic government and free market economics. However, the Americans had no intention of doing anything more than making speeches condemning the invasion of Czechoslovakia. This was because they had already accepted the right of the Soviets to assert their control over Eastern Europe, as shown by their muted response to the squashing of the Hungarian revolt 12 years earlier.
More important than Prague Spring in changing relations was the building of the Berlin Wall in 1961. Khrushchev wanted to stop the flow of refugees leaving East Germany for the west and he demanded that the Americans leave West Berlin. When Kennedy refused, the relationship between the Soviet Union and the USA broke down. Khrushchev decided to erect the Berlin wall and it became the most potent symbol of communist 'evil' that would last for decades. Tension increased between the superpowers and there was a stand-off between the tanks of both sides at Check Point Charlie. Nevertheless, the superpowers were not going to go to war over this issue because West Berlin itself was not being attacked.
The most likely trigger of World War Three, and thereby pushing the relationship to its lowest point, was the Cuban Missile Crisis of 1962. The Russians installed nuclear missiles on the island, which was only 90 miles away from the USA and therefore a grave threat. Kennedy declared a blockade of Cuba which would have meant sinking Soviet supply ships had they crossed the line. Fortunately, Khrushchev backed down at the last moment in a game of brinkmanship. During the crisis Kennedy had been advised to bomb or invade Cuba, and had he done so then MAD (Mutually Assured Destruction) could have resulted. The danger presented by the Cuban Missile Crisis therefore had a much greater impact on relations than either Prague Spring or the Berlin Wall.
One reason relations worsened was because of the Soviet invasion of Czechoslovakia in 1968. Warsaw Pact forces crushed Dubcek's attempt at liberalising his country, known as the 'Prague Spring', and this provoked expressions of outrage from the Americans. The event was not that significant, however, because it was merely confirmation of the Soviet Union's authority over Eastern Europe and its unwillingness to tolerate any reforms. Also, Czech resistance was swiftly crushed with relatively few casualties. The Americans had absolutely no intention of intervening and the event was soon overshadowed by other problems in the world.
A more important reason than the Soviet invasion of Czechoslovakia, for growing superpower hostility, was the Vietnam War, which was fought on a larger scale than any other war in the 1960s. By 1968 the Americans had committed half a million troops to fight Soviet backed communist forces. But at no point was the war likely to escalate into more direct confrontation between the superpowers because Vietnam was not that strategically important. Moreover, the Soviets were secretly pleased by the war because it was distracting the Americans and consuming a vast proportion of their military resources that could have been spent on US forces in Europe.
Greater than either Prague Spring or the Vietnam War, in causing relations to worsen, was the continuing nuclear arms race. Despite efforts to avoid another Cuban Missile Crisis by, for example, introducing a telephone hotline in 1963, both sides were building more and more weapons capable of blowing each other up several times over. There was also an intense technological competition, reflected in the space race in which the Americans put a man on the moon in 1969. The stakes could not have been higher and fear of nuclear Armageddon dominated government thinking in both Washington and Moscow.