What is mutually assured destruction?

Mutually assured destruction is the belief that two countries will not attack each other if each country has the ability to destroy the other with the same amount of military power. Mutually assured destruction or "MAD" is a principle from the era of the Cold War and is considered part of a US military doctrine. The policy is often applied to the use of nuclear weapons. For example, because both the United States and Russia have nuclear weapons that could annihilate large populations, neither uses them. To do so, would mean the end of both populations, and a large part of humanity.