The idea behind MAD is that if each side knows that the other side can strike back with enough force to cause unacceptable damage, then neither side will initiate a full-scale nuclear attack. This creates a balance of power in which neither side is willing to risk the consequences of launching a first strike.
The doctrine of MAD has been around since the early days of the nuclear age, and it has helped to prevent a nuclear war between the United States and the Soviet Union. However, there are some critics of the doctrine who believe that it is no longer effective in preventing nuclear war. They argue that the balance of power between the United States and the Soviet Union has changed, and that new nuclear powers, such as India and Pakistan, are now capable of launching devastating nuclear attacks.
Despite the criticisms, MAD remains the cornerstone of nuclear deterrence policy in the United States and other nuclear-armed countries. It is based on the belief that the possibility of mutual assured destruction is the best way to prevent nuclear war.