Second-strike can be used to describe a weapon system
, a national capability, or a strategy
. In all cases, it refers to the use of weapons in a retaliatory
(as opposed to preemptive
) attack. As such, it must be able not only to perform its intended operating functions, but must be able to survive the initial attack, which one presumes would consist of an attempt to destroy the second-strike weapons before they could be used.
A secure second-strike capability is essential for successful strategic deterrence such as is practiced today by the United States, Russia, England, France, Britain and China (yes, I realize that some of those are debatable). The submarine-launched ballistic missile is a classic example of such a system. The initial lower accuracy and payload of early versions of these weapons were acceptable because their deployment on submarines (in a location unknown to the enemy) made it highly likely that they would survive an initial nuclear attack. At that point, the only feasible use for them would be to destroy enemy cities (countervalue strikes), since the enemy's nuclear forces would presumably have been expended. Cities are large, soft targets, and extreme accuracy is not required.
Lack of a secure second-strike force can lead to destabilizing tendencies. If a nation feels that its nuclear weapons are not second-strike capable, i.e. they are unable to 'ride out' an attack and survive, then the pressure to use them or lose them during a crisis situation is much higher. If failure to launch first means that the weapons are useless, the state with such weapons has a strong incentive to strike at the slightest sign its enemy is preparing to do so.