MAD (Mutually Assured Destruction)
The Military doctrine of Mutual Assured Destruction (MAD) assumes that between two parties, each side has enough nuclear weaponry to destroy the other side; and that either side, if attacked for any reason by the other, would retaliate without fail with equal or greater force. The expected result is an immediate irreversible escalation of hostilities resulting in both combatants' mutual, total and assured destruction. It is assumed in theory of International Relations that due to this expected result of MAD, 2 nations of Nuclear weaponary would hesitate while attacking the other Nation.
The big Example of this doctrine was seen at the time of Cold war between US and Russia. It was assumed that MAD was one of the primary reasons why both parties did not escalated the war to a Nuclear level. The same logic is often used while lookign at the cases of India and Pakistan also.