A commonly held theory and an often-used argument in favour of the possession of nuclear weapons is that it deters foreign states from military threats against vital interests. For example, see deterrence theory on Wikipedia, which has plenty of references to further sources. As a more specific example, UK government policy states:
UK nuclear deterrence policy consists of 5 main principles:
preventing attack - the UK’s nuclear weapons are not designed for military use during conflict but instead to deter and prevent nuclear blackmail and acts of aggression against our vital interests that cannot be countered by other means
which means that the UK government expects that they do have a deterrent effect.
Is there any quantitative evidence that a nuclear capability deters foreign states from threatening vital interests?
Answers should refer to studies that:
- Quantitatively compare military aggression (threats or actual violence) to vital interests of nuclear vs. non-nuclear states
- Quantitatively compare military aggression (threats or actual violence) to vital interests of states before or after they became nuclear powers.
Vital interests would be interests that threaten the functioning of a state. For example, the government would be a vital interest. A vital interest is not a small outlying territory, such as the UK's Falkland Islands.
The Wikipedia list of nuclear states includes major military powers such as the USA, China, Russia, but also lesser powers such as India, Pakistan, and North Korea. Other major powers, such as Germany and Japan, do not have (their own) nuclear weapons.