For decades, military planners in the United States, former Soviet Union, and the other nuclear weapons states have convinced themselves that their nuclear weapons are a deterrent. The nuclear annihilation that would follow if these weapons were used was supposed to make any enemy stop, think, and decide that war was not worth the consequences. To make sure that an enemy had no doubt about these consequences, all the nuclear weapons states created nuclear arsenals designed to fight a nuclear war. Nuclear deterrence was built on assuming that one day the simple fear of nuclear weapons would not be enough and the weapons would have to be used.
The reliance on nuclear weapons that could be used in a real war led each nuclear weapons state to live in perpetual fear of a surprise attack that would make their weapons useless. This fear was greatest during the Cold War, when each side thought the other could not be trusted. The US and Soviet Union addressed their fears by building enormously complex early warning systems that would let them know they were about to be attacked and give them time to launch their nuclear weapons before they were destroyed.
The early warning systems of the superpowers had another crucial role. Since any war would have meant nuclear war, both sides wanted to make sure that war did not begin by accident. Early warning systems created time during which people could make decisions using real information about what was actually happening rather than responding simply on the basis of fears of what might be about to happen.
The US built and still operates the biggest and most sophisticated early warning system. It is based around a missile warning system and works by collecting information from satellites that can detect the launch of missiles from another country and radars around the world that can follow the missiles to see where they are going. The information is transmitted from these satellites and radars to where it can be processed by computers and then analysed and interpreted by people. To make sure that this is done seriously and properly, this assessment is done at several places separately. If the information is determined to be reliable, it is sent to more senior people who are supposed to decide how to respond.