For most of the twentieth century, the world feared communism, its military state, and dictatorships. The United States and the USSR represented both sides of this struggle between freedom and tyranny. In the war against Hitler and his Nazi regime, the Soviet Union and the United States found themselves on the same side of the fence. Towards the end of the war, the Soviet Union advanced on the Nazis from the east while the United States advanced from the west. The big fear for much of the twentieth century was that these two forces that despised each other would eventually meet in Germany, and nobody wanted that to happen.
At the conclusion of World War II in 1945, Germany was divided into two countries with the larger western portion becoming the Federal Republic of Germany and the smaller eastern area became the German Democratic Republic (GDR). The west was then divided into three sectors to be controlled by the occupying countries at the end of the war – the US, Britain and France – while the east would be under the control of the Soviet Union. The former capital city of Berlin was located completely within the GDR and was similarly divided into sections with the eastern half of the city controlled by the Soviets and the western half by the US, Britain and France. Years later, the Soviets would build the Berlin Wall which completely surrounded the NATO side of Berlin, resulting in further isolation from both West Germany and its allies in the West.
Shortly after World War II, the U.S. found itself battling the growth of communism in the Korean War (1950-1954). Immediately after this, communism would get a foothold in North Vietnam. In 1961, the United States began significantly expanding its role in South Vietnam, becoming actively involved by 1965. An extremely unpopular war back home, troops would eventually get pulled out in 1973. Saigon would fall in 1975 when the communist North completely took control of the country. While the United States had been spending huge amounts of money in these conflicts, the Soviet Union was putting most of its resources into growing its military into the largest in the world.
The United States and its NATO allies maintained a nuclear arsenal larger than that of the Warsaw Pact during the early years of the Cold War. The United States followed a military strategy known as the Mutually Assured Destruction (MAD) doctrine. The basic premise of this doctrine was the belief that if both sides had enough nuclear weapons neither would use them out of fear of retaliation from the other. A full nuclear war would wipe out both sides, negating any advantage from the use of nuclear weapons. This nuclear dominance had shifted to the Soviets by the end of the Vietnam War, shifting the balance of power and making the threat of a nuclear war a real concern.