Sometimes single events alter the course of history; other times, a chain reaction of seemingly lesser occurrences changes the path of nations. The intense rivalry between the United States and the Soviet Union that emerged immediately after World War II (1939–45) followed the second pattern. Known as the Cold War, the rivalry grew out of mutual distrust between two starkly different societies: communist Soviet Union and the democratic West, which was led by the United States and included Western Europe. Communism is a political and economic system in which the Communist Party controls all aspects of citizens' lives and private ownership of property is banned. It is not compatible with America's democratic way of life. Democracy is a political system consisting of several political parties whose members are elected to various government offices by vote of the people. The rapidly growing rivalry between the two emerging post–World War II superpowers in 1945 would dominate world politics until 1991. Throughout much of the time, the Cold War was more a war of ideas than one of battlefield combat. Yet for generations, the Cold War affected almost every aspect of American life and those who lived in numerous other countries around the world.

The global rivalry was characterized by many things. Perhaps the most dramatic was the cost in lives and public funds. Millions of military personnel and civilians were killed in conflicts often set in Third World countries. This toll includes tens of thousands of American soldiers in the Korean War (1950–53) and Vietnam War (1954–75) and thousands of Soviet soldiers in Afghanistan. National budgets were stretched to support the nuclear arms races, military buildups, localized wars, and aid to friendly nations. On the international front, the United States often supported oppressive but strongly anti-communist military dictatorships. On the other hand, the Soviets frequently supported revolutionary movements seeking to overthrow established governments. Internal political developments within nations around the world were interpreted by the two superpowers—the Soviet Union and the United States—in terms of the Cold War rivalry. In many nations, including the Soviet-dominated Eastern European countries, basic human freedoms were lost. New international military and peacekeeping alliances were also formed, such as the United Nations (UN), the North Atlantic Treaty Organization (NATO), the Organization of American States (OAS), and the Warsaw Pact.

Effects of the Cold War were extensive on the home front, too. The U.S. government became more responsive to national security needs, including the sharpened efforts of the Federal Bureau of Investigation (FBI). Created were the Central Intelligence Agency (CIA), the National Security Council (NSC), and the Department of Defense. Suspicion of communist influences within the United States built some individual careers and destroyed others. The national education priorities of public schools were changed to emphasize science and engineering after the Soviets launched the satellite Sputnik, which itself launched the space race.

What would cause such a situation to develop and last for so long? One major factor was mistrust for each other. The communists were generally shunned by other nations, including the United States, since they gained power in Russia in 1917 then organized that country into the Soviet Union. The Soviets' insecurities loomed large. They feared another invasion from the West through Poland, as had happened through the centuries. On the other hand, the West was highly suspicious of the harsh closed society of Soviet communism. As a result, a move by one nation would bring a response by the other. Hard-liners on both sides believed long-term coexistence was not feasible.

A second major factor was that the U.S. and Soviet ideologies were dramatically at odds. The political, social, and economic systems of democratic United States and communist Soviet Union were essentially incompatible. Before the communist (or Bolshevik) revolution in 1917, the United States and Russia competed as they both sought to expand into the Pacific Northwest. In addition, Americans had a strong disdain for Russian oppression under their monarchy of the tsars. Otherwise, contact between the two growing powers was almost nonexistent until thrown together as allies in a common cause to defeat Germany and Japan in World War II.

It was during the meetings of the allied leaders in Yalta and Potsdam in 1945 when peaceful postwar cooperation was being sought that the collision course of the two new super-powers started becoming more evident. The end of World War II had brought the U.S. and Soviet armies face-to-face in central Europe in victory over the Germans. Yet the old mistrusts between communists and capitalists quickly dominated diplomatic relations. Capitalism is an economic system in which property and businesses are privately owned. Prices, production, and distribution of goods are determined by competition in a market relatively free of government intervention. A peace treaty ending World War II in Europe was blocked as the Soviets and the U.S.-led West carved out spheres of influence. Western Europe and Great Britain aligned with the United States and collectively was referred to as the "West"; Eastern Europe would be controlled by the Soviet Communist Party. The Soviet Union and its Eastern European satellite countries were collectively referred to as the "East." The two powers tested the resolve of each other in Germany, Iran, Turkey, and Greece in the late 1940s.

In 1949, the Soviets successfully tested an atomic bomb and Chinese communist forces overthrew the National Chinese government, and U.S. officials and American citizens feared a sweeping massive communist movement was over-taking the world. A "red scare" spread through America. The term "red" referred to communists, especially the Soviets. The public began to suspect that communists or communist sympathizers lurked in every corner of the nation.

Meanwhile, the superpower confrontations spread from Europe to other global areas: Asia, Africa, the Middle East, and Latin America. Most dramatic were the Korean and Vietnam wars, the Cuban Missile Crisis, and the military standoffs in Berlin, Germany. However, bloody conflicts erupted in many other areas as the United States and Soviet Union sought to expand their influence by supporting or opposing various movements.

In addition, a costly arms race lasted decades despite sporadic efforts at arms control agreements. The score card for the Cold War was kept in terms of how many nuclear weapons one country had aimed at the other. Finally, in the 1970s and 1980s, the Soviet Union could no longer keep up with the changing world economic trends. Its tightly controlled and highly inefficient industrial and agricultural systems could not compete in world markets while the government was still focusing its wealth on Cold War confrontations and the arms race. Developments in telecommunications also made it more difficult to maintain a closed society. Ideas were increasingly being exchanged despite longstanding political barriers. The door was finally cracked open in the communist European nations to more freedoms in the late 1980s through efforts at economic and social reform. Seizing the moment, the long suppressed populations of communist Eastern European nations and fifteen Soviet republics demanded political and economic freedom.

Through 1989, the various Eastern European nations replaced long-time communist leaders with noncommunist officials. By the end of 1991, the Soviet Communist Party had been banned from various Soviet republics, and the Soviet Union itself ceased to exist. After a decades-long rivalry, the end to the Cold War came swiftly and unexpectedly.

A new world order dawned in 1992 with a single superpower, the United States, and a vastly changed political landscape around much of the globe. Communism remained in China and Cuba, but Cold War legacies remained elsewhere. In the early 1990s, the United States was economically burdened with a massive national debt, the former Soviet republics were attempting a very difficult economic transition to a more capitalistic open market system, and Europe, starkly divided by the Cold War, was reunited once again and sought to establish a new union including both Eastern and Western European nations.