Producing, accumulating and deploying long-range nuclear armaments by which countries can threaten another without using the territory of their allies. Horizontal proliferation is the geographical spread of nuclear weapons. Vertical proliferation is the development and increasing sophistication of existing sophistication of existing nuclear arsenals.
One can identify three schools of thought as to the purpose of nuclear weapons. Firstly, that which believes they serve to deter war and, therefore, that as long as there is a prospect of war nuclear weapons remain essential. Secondly, that which believes that the utility of nuclear weapons is restricted to the deterrence of other nuclear weapons and, therefore, that global elimination is logical, feasible and desirable if undertaken collectively. Thirdly, there is the school that believes the use or even threat of use of nuclear weapons can never be justified and, therefore, that they should be abandoned forthwith.
The five declared nuclear weapons states are the USA, UK, France, China, and former USSR (warheads in Russia, Belarus, Kazakhstan and Ukraine). In addition there are de facto nuclear weapons states (those with acknowledged weapons-producing capability and/or unknown numbers of accumulated weapons, such as India, Israel, Pakistan, South Africa and Iran) and threshold states (such as Iraq, North Korea, Syria, Argentina and Brazil). Following the end of the Cold War, there is a new kind of strategic arms race developing in this nuclear weapons club, unaffected by the reduction in the size of arsenals in the declared weapons states.
The proliferation of missiles is accelerating in the third world. UK and French plans are to expand their arsenals; cuts in the Chinese arsenal are considered unlikely. The UK, for one, is moving its strategic orientation away from Russia and back to the rest of the world. As an example, it is likely that some Trident missiles would be armed with single low-yield warheads to make them usable in "shot across the bow" operations in a nuclear crisis with a power like in the Middle East.
All military capabilities have a role to play in preventing war. Both nuclear and conventional forces therefore contribute to deterrence, providing a credible range of options for responding proportionately to an aggressor's behaviour.
Nuclear deterrence still has an important contribution to make in insuring against the re-emergence of major strategic military threats, in preventing nuclear coercion, and in preserving peace and stability.
Ultimately, the former Soviet Union and the USA understood that the political, economic, and even security costs of long-term reliance on nuclear deterrence are greater than the advantages. The revolution that both these superpowers underwent at the end of the 1980s was not a technological revolution but a conceptual one, in part based on better knowledge of the psychosocial processes by which certain technologies become outmoded and delegitimized. Similar cognitive changes caused Argentina and Brazil, states with nuclear capacity, to come to the brink of developing nuclear-weapons and then avoid it.