Fail-deadly is a concept in nuclear military strategy which encourages deterrence by guaranteeing an immediate, automatic and overwhelming response to an attack. The term fail-deadly was coined as a contrast to fail-safe.
It is an example of second strike strategy, in that aggressors are discouraged from attempting a first strike 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 by a first strike. The deterrent efficacy of such a system clearly depends on other nuclear-armed nations having foreknowledge of it. The Soviet Union used a fail-deadly system known as Dead Hand (codenamed "Perimetr"); it is not certain if Russia still uses it. Such a system served as a main plot element of Stanley Kubrick's 1964 film Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb.
Fail-deadly is also associated with "massive retaliation", a deterrence strategy which ensures that the counter strike will be conducted on a larger scale than the initial attack. If an aggressor launched one missile at another party, they would get ten in return. If they launched ten, they would receive 100, and so on.
A more prosaic example of a fail-deadly instrument is a switch which must be constantly held to prevent the triggering of an explosive, which ensures that a suicide bombing is not prevented by killing the person with the bomb. Examples of this are the safety lever of a hand grenade, or the detonator of the large chemical bomb used in Terminator 2: Judgement Day. Another example of the fail-deadly is the push-button switch held down by the suicide bomber in the opening scene of the 2007 in film film Rendition, in which the suicide bomber is shot by his handlers to ensure his hand releases the button and triggers the bomb. This is an alteration of a device known as a dead man's switch, which is designed to save lives in the event of operator incapacitation.