How much danger would Iranian nuclear weapons pose to the world? This question animates the debate over whether the threat of the Iranian nuclear program is worth robust sanctions or a preventive military attack. Nuclear weapons are by their nature alarming, and the Iranian regime says and does a lot of alarming things. But how useful are nuclear weapons, even to a state with bad intentions? How much do they change tactical and strategic behavior? For devices capable of destroying cities and killing millions, the answer is surprisingly murky.
States sometimes have excellent reasons for developing nuclear weapons. Countries that believe themselves likely to have trouble deterring potential conventional foes consider nuclear weapons an insurance policy against catastrophic defeat. Although nukes cannot prevent small-scale conventional defeats, they can presumably limit the damage, especially when regime survival is at stake. We can identify several situations in which nuclear weapons probably had an impact on the outcomes of crises between states. Nuclear weapons may have limited the extent of the Kargil War between Pakistan and India in 1999, and they may have prevented India from launching a conventional retaliation for the Mumbai attacks in 2009. The effect of nuclear deterrence in the Cold War is hard to calculate -- the closest the U.S. and the USSR came to war was over new deployments of nuclear weapons -- but the presence of massive, second-strike arsenals on each side may well have served to reduce or at least contain tensions.
But states and policymakers habitually overestimate the impact of nuclear weapons. This happens among both proliferators and anti-proliferators. Would-be proliferators seem to expect that possessing a nuclear weapon will confer “a seat at the table” as well as solve a host of minor and major foreign policy problems. Existing nuclear powers fear that new entrants will act unpredictably, destabilize regions and throw existing diplomatic arrangements into flux. These predictions almost invariably turn out wrong; nuclear weapons consistently fail to undo the existing power relationships of the international system.