One of the arguments that is often invoked to justify a hardline approach to Iran's nuclear program is the fear of a "regional arms race." In this view, if Iran were to get the bomb, neighboring states like Saudi Arabia and Egypt might be forced to get them too, with presumably harmful effects on regional stability. (For a recent invocation of this line of thinking, see Michael Slackman's article in the New York Times today).
One obviously cannot rule out such a possibility out, but there are good reasons not to accept this particular justification uncritically. To begin with, the real danger is not a regional arms race per se, it is the possibility that an arms race might lead to conflict in a critical region (or make it easier for terrorists to steal a weapon). By themselves, arms races just waste money, which is obviously not a good thing but not necessarily a disaster in strategic terms. So the question is two-fold: Would an arms race actually occur if Iran went nuclear, and would it then have dangerous effects on regional stability?
Here the evidence is mixed. With respect the first question, history suggests that one state's acquisition of nuclear weapons does not necessarily produce an immediate flock of imitators. The Soviet Union did get nuclear weapons because the United States had them, and one could argue that Soviet acquisition (and the desire to retain the trappings of great power status) played a role in the British and French decisions to go nuclear in the 1950s. China's decision to get a minimum deterrent of its own undoubtedly reflected their concerns about U.S. (and later, Soviet) power, although Mao seems to have been as worried by the other great powers’ conventional capabilities as by their nuclear arsenals. And it's clear that Pakistan's nuclear program was an obvious response to India's nuclear program. So we obviously cannot rule out the possibility that an Iranian bomb would encourage others to follow suit.
But the overall record on this point is far from clear. There are between 40 and 60 states with the technological capacity and economic wherewithal to build a nuclear bomb, and the vast majority of them have decided not to do so, even when there were other nuclear powers in their neighborhood. A few states have started down that road and then turned back, sometimes in the face of international pressure (Libya, Brazil, Argentina), and sometimes mostly on their own (Sweden, South Africa). Israel's acquisition of nuclear weapons in the 1960s did spark some interest in the Arab world, but only Saddam Hussein got really serious about it (and even he gave up trying for nukes in the 1990s). Libya had a semi-serious nuclear program too, but it was hardly a crash program and Ghaddafi eventually abandoned it as well. Iran’s own nuclear program (which began under the Shah) reflected broader security concerns and the Shah's own desire for status, and doesn't appear to have been a direct response to anyone else's bomb. North Korea’s entry into the nuclear club hasn't led South Korea, Japan, or anyone else to start a new nuclear weapons program yet. In short, people have been forecasting the rapid proliferation of nuclear weapons ever since the nuclear age began, but all of those forecasts have been overly pessimistic.
The key point to remember is that a decision to build a bomb involves some complex cost-benefit calculations, and Iran's acquisition of a nuclear weapon would not necessarily lead any of its neighbors to decide that their best course is to follow suit. One reason they might hold back is simply the recognition that getting a bomb would not enhance Iran's influence as much as is sometimes claimed. China did not suddenly become a more influential power when it tested a bomb in 1964; its rise to true great power status came when it began to modernize its economy in the l980s. Getting a bomb may have reinforced Israel's "existential security" (which is why Ben Gurion wanted one), but having a couple of hundred nuclear weapons doesn’t enable them to blackmail the Palestinians or the other Arab states into doing whatever Jerusalem wants. Similarly, North Korea has hardly any influence in world affairs despite its recent entry into the nuclear club; the only thing that that Pyongyang can do with its weapon is discourage others from putting too much pressure on them. Americans really should understand this: we have several thousand nuclear weapons and we have a tough enough time getting other states -- even rather weak ones -- to do what we want. The same would be true for a nuclear Iran: it could not blackmail anyone because the threat would not be credible, and even nearby states might find it easier to adjust to than we sometimes think .
By the way, this same logic may also help convince Iran that it doesn't need to go all the way to full acquisition of a nuclear capability. It won't buy them much influence, but it still might encourage some of their neighbors to follow suit. Ironically, that situation might decrease Iran’s regional influence over time. Iran is the most populous state in the Gulf region, and it has enormous economic potential. If the mullahs ever get their act together, Iran’s conventional capabilities would overshadow the other states in the region. And if that's the case, crossing the nuclear threshold might lead others to look for a cheap way to counter that. Thus, from Iran's own point of view, staying on this side of the nuclear threshold (but having the capacity to go nuclear quickly if need be), might be the optimal strategy, particularly if they were less worried about an imminent Israeli or U.S. attack.
Next, would a Middle Eastern arms race lead to war? There is a vast academic literature on the general relationship between arms races and war, and the results are at best inconclusive. The empirical results are highly sensitive to the model specification and other definitional questions, and the best short answer is that the effect is highly conditional: arms races may raise the danger of war in some circumstances, but make war less likely in others.
And what about nuclear arms races? Here too, there is a heated academic debate. On one side are those who believe the slow spread of nuclear arsenals might actually be stabilizing (or at least not destabilizing), essentially because the logic of deterrence would kick in, make war too dangerous, and also induce greater overall caution short of war. Other scholars question this optimistic appraisal, and argue that new nuclear states might have trouble establishing stable deterrent relationships and would also create a greater risk of nuclear leakage to terrorists.
I lean toward the former view, but it’s clearly not an open-and-shut case. Nonetheless, it is worth noting that the spread of nuclear weapons has generally been stabilizing so far; in the sense that no one has launched a major war of aggression against a nuclear power at any time in the past. (The Egyptian-Syrian attack on Israel in the 1973 October War is not an exception, by the way, as it was clearly a "limited aims" attack focused on regaining territory captured by Israel in 1967, and not an attempt to conquer Israel itself). The Kargil conflict between India and Pakistan is probably the most serious counter-example, but even that dispute remained at a fairly low level and fear of nuclear escalation probably played some role in allowing cooler heads to prevail.
There are plenty of good reasons to try to prevent Iran from going nuclear, which is why one hopes that the talks in Geneva will make progress. But the sometimes apocalyptic visions of what an Iranian bomb might mean rest on worst-case arguments about which one should maintain a healthy skepticism. And for other reasons to be skeptical about the current effort to mobilize for war, see Juan Cole's "top ten things you know about Iran that are not true," here.
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Stephen M. Walt is the Robert and Renée Belfer professor of international relations at Harvard University.