Massive retaliation, also known as a massive response or massive deterrence, is a military doctrine

Military doctrine

Military doctrine is the concise expression of how military forces contribute to campaigns, major operations, battles, and engagements.It is a guide to action, not hard and fast rules. Doctrine provides a common frame of reference across the military...

 and nuclear strategy

Nuclear strategy

Nuclear strategy involves the development of doctrines and strategies for the production and use of nuclear weapons.As a sub-branch of military strategy, nuclear strategy attempts to match nuclear weapons as means to political ends...

 in which a state commits itself to retaliate in much greater force in the event of an attack.
In the event of an attack from an aggressor, a state would massively retaliate by using a force disproportionate to the size of the attack.

The aim of massive retaliation is to deter

Deterrence theory

Deterrence theory gained increased prominence as a military strategy during the Cold War with regard to the use of nuclear weapons, and features prominently in current United States foreign policy regarding the development of nuclear technology in North Korea and Iran. Deterrence theory however was...

 an adversary from initially attacking.