The Nuclear Bomb
The United States was the first to develop nuclear weapons through the Manhattan Project during World War II. The US ended the war with Japan by dropping nuclear bombs on the cities of Hiroshima and Nagasaki.
Nuclear bombs are extremely powerful weapons that can destroy an entire city and kill tens of thousands of people. The only time nuclear weapons have been used in war was at the end of World War II against Japan. The Cold War was predicated on the fact that neither side wanted to engage in a nuclear war that could destroy much of the civilized world.
Start of the Arms Race
On August 29, 1949 the Soviet Union successfully tested its first atomic bomb. The world was shocked. They did not think the Soviet Union was this far along in their nuclear development. The Arms Race had begun.
In 1952 the United States detonated the first hydrogen bomb. This was an even more powerful version of the nuclear bomb. The Soviets followed up by exploding their first hydrogen bomb in 1953.
ICBMs
In the 1950s both countries worked on developing Intercontinental Ballistic Missiles or ICBMs. These missiles could be launched from long range, as far away as 3,500 miles.
Defense
As both sides continued to develop new and more powerful weapons, the fear of what would happen if war broke out spread throughout the world. Militaries began to work on defenses such as large radar arrays to tell if a missile had been launched. They also worked on defense missiles that could shoot down ICBMs.
At the same time people built bomb shelters and underground bunkers where they could hide in the case of nuclear attack. Deep underground facilities were built for high ranking government officials where they could reside safely.
Mutual Assured Destruction
One of the major factors in the Cold War was termed Mutual Assured Destruction or MAD. This meant that both countries could destroy the other country in the case of attack. It wouldn't matter how successful the first strike was, the other side could still retaliate and destroy the country which first attacked. For this reason, neither side ever used nuclear weapons. The cost was too high.