When the first atomic bomb was exploded over New Mexico in 1945, the scientists working on the Manhattan Project believed their work had made the United States immune to attack. Then, the Soviet Union began developing its own nuclear weapons and building a network of missiles capable of carrying them to the United States. American military planners quickly came to believe that, unless the United States could be destroyed in a first strike, there was little point in launching a full-scale war against an enemy with nuclear capabilities.
They developed a doctrine called MAD, or mutually assured destruction. It held that each side had the potential to destroy a large proportion of the population and infrastructure of the other, and that such an exchange would probably result in nuclear annihilation of both sides. This theory served as the basis for the policies of deterrence that became the modus operandi of superpowers during the Cold War.
A few people outside of the radius of destruction might survive, but they would be mortally burned or blinded from thermal flash burns; crushed by debris or lost in collapsed buildings; cut by glass splinters; and dead from exposure to radiation. They might also be trapped by contaminated water or food shortages and the breakdown of emergency services.
The impact of nuclear weapons on people has been studied for decades. A few principles are clear: Shielding, distance and time can limit the harm from a disaster. By staying as far away from the blast and fallout as possible, avoiding windows and doors, and turning off air conditioners and heaters, a person can lower their risk of radiation exposure.