Fail-deadly is a concept in military strategy which encourages deterrence by guaranteeing an immediate, automatic and overwhelming response to an attack. Under Fail-deadly nuclear deterrence, policies and procedures controlling the retaliatory strike will authorize launch even if the existing command and control structure has already been neutralized.

Fail-deadly is also associated with Massive Retaliation, a deterrence strategy which ensures that the counter strike will be conducted by a higher factor. Basically, if an aggressor launched one missile at another party, they would get 10 in return. If they launched 10, they would get 100 in return, and so on.

A more prosaic example of a Fail-deadly instrument is a dead man's switch which must be constantly held to prevent the triggering of an explosive, which has the purpose to make sure that a suicide bombing is not prevented by killing the person with the bomb.

The term Fail-deadly was coined as a contrast to Fail-safe.

See also

• Mutual assured destruction