A nuclear close call is an incident that could lead to at least one unintended nuclear detonation/explosion. These incidents typically involve a perceived imminent threat to a nuclear-armed country which could lead to retaliatory strikes against the perceived aggressor. The damage caused by international nuclear exchange is not necessarily limited to the participating countries, as the hypothesized rapid climate change associated with even small-scale regional nuclear war could threaten food production worldwide—a scenario known as nuclear famine. Despite a reduction in global nuclear tensions after the end of the Cold War, estimated nuclear warhead stockpiles total roughly 15,000 worldwide, with the United States and Russia holding 90% of the total. Although exact details on many of these nuclear close calls are hard to come by, the analysis of particular cases has highlighted the importance of a variety of factors in preventing accidents. At an international level, this includes the importance of context and outside mediation; at the national level, effectiveness in government communications, and involvement of key decision-makers; and, at the individual level, the decisive role of individuals in following intuition and prudent decision-making, often in violation of protocol.
It is critical to note that there have been several instances where nuclear war has been a possibility and we cannot look at these instances as simply being rationally avoided. The role of luck has also been key in cases where ultimately the right decision was made due to wrong information or ignorance. Our knowledge about nuclear weapons safety and command and control are limited and also reflect why the retrospective analysis of nuclear close calls doesn’t depict the whole truth. The vulnerabilities of our knowledge about control and safety of nuclear weapons must be a variable in understanding the occurrence of close calls and how we proceed to analyze them. As Scott Sagan has argued about the Cuban missile crisis, "Many serious safety problems, which could have resulted in an accidental or unauthorized detonation or a serious provocation to the Soviet government, occurred during the crisis. None of these incidents led to inadvertent escalation or an accidental war. All of them, however, had the potential to do so." Therefore, the record of absence of war that has been maintained should not be reduced to simply good management of threats.
The most common mistakes made about the learning of nuclear close calls are that it's adequate, complete, and unanimous and it’s important to question all three assumptions. Learning about the Cuban missile crisis cannot be regarded as adequate because of the key role played by luck. Luck is too often taken as a confirmation that nuclear deterrence kept the peace but luck should not be misread as successful deterrence. There were instances during the Cuban missile crisis where lack of information, misperception, and ideology could have led to disaster had luck not played the role that it did, including one that former US Secretary of Defense Robert McNamara has been vocal about. Including luck in a successful deterrence strategy, therefore, acts as both a conceptual confusion and a retrospective illusion which has also prevented an adequate learning about the Cuban missile crisis. The learning about the Cuban missile crisis is not complete because the United States is the only country that has been transparent. Efforts to declassify information are not linear and attempts at reclassification sporadic. Additionally, probability is only part of the risk equation. For some, the high consequences of a nuclear detonation will always be too high a risk. How close is “too close for comfort”?' and ‘at what point would the risk be assessed as acceptable or comfortable?’ are two important interconnected questions for two reasons: the abstract nature of the nuclear threat and its devastating consequences are fuelling the temptation to ignore it, and the rhetoric of deterrence is often framed as defensive and seemingly risk-free. The learning about the Cuban missile crisis is not unanimous. Different countries had different experiences of the crisis although it is still considered to be the single most devastating event in nuclear history. The opening of archives worldwide after fifty years suggests that this diversity of interpretations of the crisis is going to remain and possibly increase, without exposing as an illusion the belief that we have already learned what we need to in order to successfully deal with similar events in the future. Documents in those archives suggest that there existed very diverse experiences and interpretations of nuclear vulnerability during the crisis, which are likely to lead to very different conclusions. For example, in the United Kingdom, the elites were well aware of their nuclear vulnerability at the time and saw the possibility of escalation to an all-out nuclear war while the French barely experienced fear of nuclear war.