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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 be tempted 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 to not 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, allowing for nuclear retaliation to be carried out. The nuclear triad is a way for countries to diversify their nuclear arsenals 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 it needs 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 are 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. Some nuclear missiles are located near borders of armed countries in order to dissuade attackers; an attack on these sites would likely start a war against multiple countries at once.
Launch on warning is a strategy of nuclear weapon retaliation that gained recognition during the Cold War between the United States and the Soviet Union. In addition to the nuclear triad, nations deploy an early warning system that detects incoming nuclear missiles. This gives that nation the capability and option to launch a retaliatory second strike before the incoming nuclear first strike hits any of its targets. This is another method of solidifying second-strike capabilities and deterring a first strike from another nuclear power.
As early as 1940, science fiction writer Robert A. Heinlein wrote Solution Unsatisfactory in which he described a nuclear arms race between the United States and the Soviet Union. In one episode, the US cabinet discusses the scenario of a Soviet surprise attack in which American cities would be destroyed, but the US armed forces would survive and launch a counter-attack.
- Enrico Fels (2008): Will the Eagle strangle the Dragon? As Assessment of the U.S. Challenges towards China’s Nuclear Deterrence, Trends East Asia Studie Nr. 20 (February 2008).