Despite the foundation of the UN, and the fact that both the US and USSR were permanent members of the Security Council, the divide between them undermined the possibility of global unity. Instead, by the late 1940s, the world was increasingly split into the two rival “camps” of the Cold War. The term itself refers to the decades-long rivalry between the two postwar “superpowers,” the United States and the Soviet Union. This was a conflict that, fortunately for the human species, never became a "hot" war. Both sides had enormous nuclear arsenals by the 1960s that would have ensured that a "hot" war would almost certainly see truly unprecedented destruction, up to and including the actual possibility of the extinction of the human species (the American government invented a memorable phrase for this known as M.A.D.: Mutually Assured Destruction). Instead, both nations had enough of a collective self-preservation instinct that the conflict worked itself out in the form of technological and scientific rivalry, an enormous and ongoing arms race, and "proxy wars" fought elsewhere that did not directly draw both sides into a larger conflict.
The open declaration of Cold War, as it were, consisted of doctrines and plans. In 1947 the US issued the Truman Doctrine, which pledged to help people resist communism wherever it appeared – the rhetoric of the doctrine was about the defense of free people who were threatened by foreign agents, but as became very clear over the next few decades, it was more important that people were not communists than they were “free” from dictatorships. The Truman Doctrine was born out of the idea of “containment,” of keeping communism “contained” to the countries it already had taken over. The immediate impetus for the doctrine was a conflict raging in Greece after WWII, in which the communist resistance movement that had fought the Nazis during the war sought to overthrow the right-wing, royalist government of Greece in the aftermath. Importantly, while both the British and then the US supported the Greek government, the USSR did not lend any aid to the communist rebels, rightly fearing that doing so could lead to a much larger war. Furious at what he regarded as another instance of western capitalist imperialism, however, Stalin pulled the USSR out of the Bretton Woods economic agreement in early 1948.
The Truman Doctrine was closely tied to the fear of what American policy-makers called the “Domino Theory”: if one nation “fell” to communism, it was feared, communism would spread to the surrounding countries. Thus, preventing a communist takeover anywhere, even in a comparatively small and militarily insignificant country, was essential from the perspective of American foreign policy during the entire period of the Cold War. That theory was central to American policy from the 1950s through the 1980s, deciding the course of politics, conflicts, and wars from Latin America to Southeast Asia.
Along with the Truman Doctrine, the United States introduced the Marshall Plan in 1948, named for the American secretary of state at the time. The Marshall Plan consisted of enormous American loans to European countries trying to rebuild from the war. European states also founded a intra-European economic body called the Organization of European Economic Cooperation that any country accepting loans was obliged to join. Stalin regarded the OEEC as a puppet of the US, so he banned all countries under Soviet influence from joining, and hence from accepting loans. Simultaneously, the Soviets were busy extracting wealth and materials from their new puppets in Eastern Europe to help recover from their own war losses. The legacy of the Marshall Plan, the OEEC, and Soviet policy was to create a stark economic division between west and east: while Western Europe rapidly recovered from the war, the east remained poor and comparatively backwards.
By 1948, in the words of Winston Churchill, the “iron curtain” had truly fallen across Eastern Europe. Everywhere, local communist parties at first ruled along with other parties, following free elections. Then, with the aid of Soviet “advisers,” communists from Poland to Romania pushed other parties out through terror tactics and legal bans on non-communist political organizations. Soon, each of the Eastern European states was officially pledged to cooperation with the USSR. Practically speaking, this meant that every Eastern European country was controlled by a communist party that took its orders directly from Moscow - there was no independent political decision-making allowed.
The major exception was Yugoslavia. Ironically, the one state that had already been taken over by a genuine communist revolution was the one that was not a puppet of the USSR. During the war, an effective anti-German resistance was led by Yugoslav communists, and in the aftermath they succeeded in seizing power over the entire country. Tito, the communist leader of Yugoslavia, had great misgivings about the Soviet takeover of the rest of Eastern Europe, and he and Stalin angrily broke with one another after the war. Thus, Yugoslavia was a communist country, but not one controlled by the USSR.
In turn, it was Stalin's anger that Yugoslavia was outside of his grasp that inspired the Soviets to carry out a series of purges against the communist leadership of the Eastern European countries now under Soviet domination. Soviet agents sought “Titoists” who were supposedly undermining the strength of commitment to communism. Between 1948 and 1953 more communists were killed by other communists than had died at the hands of the Nazis during the war (i.e. in terms of direct Nazi persecution of communists, not including casualties of World War II itself). Communist leaders were put on show trials, both in their own countries and sometimes after being hauled off to Moscow, where they were first tortured into confessing various made-up crimes (collaborating with western powers to overthrow communism was a popular one), then executed. It is worth noting that this period, especially the first few years of the 1950s, saw anti-Semitism become a staple of show trials and purges as well, as latent anti-Semitic sentiments came to the surface and Jewish communist leaders suffered a disproportionate number of arrests and executions, often accused of being “Zionists” secretly in league with the west.
Simultaneously, the world was dividing into the two “camps” of the Cold War. The zones of occupation of Germany controlled by the US, France, and Britain became the new nation of the Federal Republic of Germany, known as West Germany, while the Soviet-controlled zone became the German Democratic Republic, or East Germany. Nine western European countries joined the US in forming the North Atlantic Treaty Organization, NATO, whose stated purpose was the defense of each of the member states from invasion – understood to be the invasion of Western Europe by the USSR. The USSR tested its first atomic bomb in August of 1949, thereby establishing the stakes of the conflict: the total destruction of human life. And, finally, by 1955 the Soviets had formalized their own military system with the Warsaw Pact, which bound together the Soviet Bloc in a web of military alliances comparable to NATO.
In hindsight, it is somewhat surprising that the USSR was not more aggressive in the early years of the Cold War, making no overt attempts to sponsor communist takeovers outside of Eastern Europe. The one direct confrontation between the two camps took place between May of 1948 and June of 1949, when the Soviets blockaded West Berlin. The city of Berlin was in East Germany, but its western “zones” remained in the hands of the US, Britain, and France, the phenomenon a strange relic of the immediate aftermath of the war. As Cold War tensions mounted, Stalin ordered the blockade of all supplies going to the western zones. The US led a massive ongoing airlift of food and supplies for nearly a year while both sides studiously avoided armed confrontation. In the end, the Soviets abandoned the blockade, and West Berlin became a unique pocket of the western camp in the midst of communist East Germany.
It is worth considering the fact that Europe had been, scant years earlier, the most powerful region on Earth, ruling the majority of the surface of the globe. Now, it was either under the heel of one superpower or dominated by the other, unable to make large-scale international political decisions without implicating itself in the larger conflict. The rivalries that had divided the former “great powers” in the past seemed insignificant compared to the threat of a single overwhelming war initiated by foreign powers that could result in the end of history.