Second-strike capability

In nuclear strategy, a second-strike capability is a country's assured ability to respond to a nuclear attack with powerful nuclear retaliation against the attacker. To have such an ability (and to convince an opponent of its viability) is considered vital in nuclear deterrence, as otherwise the other side might attempt to try to win a nuclear war in one massive first strike against its opponent's own nuclear forces.

The possession of second-strike capabilities counters a first-strike nuclear threat and can support a no first use nuclear strategy. Reciprocal second-strike capabilities usually cause a mutual assured destruction defence strategy, though one side may have a lower level minimal deterrence response.

Second-strike capabilities can be further strengthened by implementing fail-deadly mechanisms. These mechanisms create a threshold and guaranteed consequences if that threshold is breached. For instance, a threshold may be for an allied nation not to be attacked. If a rival nation then breaches this threshold by attacking the allied nation, then the predetermined consequences for this action go into effect. These predetermined consequences could include a wide range of responses, including a retaliatory nuclear second strike.

The crucial goal in maintaining second-strike capabilities is preventing first-strike attacks from taking out a nation's nuclear arsenal. In this manner, a country can carry out nuclear retaliation even after absorbing a nuclear attack. The United States and other countries have diversified their nuclear arsenals through the nuclear triad in order to better ensure second-strike capability.

Submarine-launched ballistic missiles are the traditional, but very expensive, method of providing a second strike capability, though they need to be supported by a reliable method of identifying who the attacker is. Using SLBMs as a second-strike capability has a serious problem, because in retaliation for a submarine-launched ICBM, the wrong country could be targeted, and can cause a conflict to escalate. However, implementation of second strikes is crucial to deter a first strike. Countries with nuclear weapons make it their primary purpose to convince their opponents that a first strike is not worth facing a second strike. Such countries have many diverse launch mechanisms, prepared responses to various nuclear attack scenarios, launch mechanisms in many different areas of the country, and underground launch facilities that are specifically designed to withstand a nuclear attack.

...
Wikipedia