It is almost 65 years since the development of the first nuclear bomb, and yet we have had only two cases of use of nuclear weapons in war, namely Hiroshima and Nagasaki. So we have been spared the horror of a large nuclear war during this period when more than 130,000 nuclear weapons were built. This is a very unusual event in the history of mankind: so many weapons built, never to be used. Why has this happened? First, the leadership of the two nuclear superpowers and of the smaller nuclear States behaved as rational decision makers, as far as the control of nuclear weapons and the decision not to initiate their use were concerned. In others words, deterrence worked, but we have to recall that the Cuban missile crisis of 1962 and other lesser crises pushed the risk of a nuclear confrontation very close to the abyss. Moreover, the system of nuclear deterrence worked and still works now on the basis of the capability of each nuclear superpower to react promptly if they receive information that they are under nuclear missile attack from their opponent. The idea is that each nuclear superpower should react against the opponent before its own nuclear missiles are destroyed while still on the ground or in their silos. With this system, known as nuclear reaction alert or "launch on warning", we have had numerous incidents of false attack that risked accidental nuclear war. Among the factors that spared mankind from the horror of a nuclear war, one was good luck, in not taking wrong decisions at critical moments, and in keeping technical mistakes and failures ultimately under control.
We know that the probability of having a catastrophic event depends on the number of critical events: the higher the number, the higher the probability. In our case, the probability of a nuclear conflict depends clearly on the number of crises which could possibly induce a nuclear war and on the number of technical failures of the nuclear control systems. These numbers in turn depend on the number of existing nuclear arsenals, on the number of nuclear weapons in those arsenals, and on the number of people who have access to the nuclear button.
In avoiding a nuclear catastrophe we have been helped by the fact that contrary to the expectations of the early nuclear age, most nations have remained non-nuclear; in other words proliferation was contained.
THE NON-PROLIFERATION REGIME
The basic instrument which helped contain the spread of nuclear weapons is the 1968 Treaty on the Non-Proliferation of Nuclear Weapons (NPT), generally considered to be the corner-stone of nuclear stability. The NPT distinguishes its parties between nuclear-weapon States (NWS) (States which conducted a nuclear test before 1967) and all the other States that, in order to be a member of the NPT, are classified as non-nuclear-weapon States (NNWS). The Treaty has basically three pillars:
The Principle of Non-Proliferation: The non-nuclear-weapon States refrain from acquiring nuclear weapons or from seeking the control of nuclear weapons, while the nuclear- weapon States agree not to transfer nuclear weapons or parts of them to others. Moreover, all Parties to the Treaty should refrain from transferring (un-safeguarded) fissile material to non-nuclear-weapon States.
The Principle of Disarmament: Parties to the Treaty, and particularly the nuclear-weapon States, commit themselves to negotiations in good faith aimed at achieving an early stage nuclear disarmament and the cessation of the nuclear arms race.
The Principle of Access to Peaceful Nuclear Technology: All Parties to the NPT have the right to develop and be assisted in the development of nuclear energy for civilian purposes.
The cold war ended with a significant effort in the direction of nuclear disarmament. Between the second half of the 1980s and the beginning of the 1990s, the United States and the Russian Federation dramatically reduced the size of their arsenals. …