Fail-deadly is a concept in nuclear military strategy that encourages deterrence by guaranteeing an immediate, automatic, and overwhelming response to an attack, even if there is no one left to trigger such retaliation.^{[citation needed]} The term fail-deadly was coined as a contrast to fail-safe.

Fail-deadly can refer to specific technology components, or the controls system as a whole. The United Kingdom's fail-deadly policies delegate strike authority to submarine commanders in the event of a loss of command (using letters of last resort), ensuring that even when uncoordinated, nuclear retaliation can be carried out.