U.S. spy planes discovered that there were Soviet missile bases being constructed in Cuba, capable of delivering nuclear warheads to every city in the United States. And, to make matters worse, there were boats carrying missiles on the way from the Soviet Union.
President John F. Kennedy ordered American ships to block the island and told the Soviet Union to turn back their missile-carrying ships. They did not.
The United States had already installed nuclear missiles in Turkey, pointed at the Soviet Union. The Soviet Union said that if the United States could do it, so could they. The boats carrying missiles did not stop.
Thousands of American troops assembled down in Florida. People all over the United States started building bomb shelters. It seemed like war was about to come. The United States was put on the highest level of alert, greater than any other time during the Cold War.
In the end, the Soviet Union backed down at the last minute. But the world had never come this close to nuclear war before. As a result, the United States and the Soviet Union signed the Nuclear Test Ban Treaty in 1963. This treaty banned the testing of nuclear bombs in the air.