Stephen Walt casts doubt on the idea that a nuclearized Iran would spark an arms race in the Middle East:
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). [...] 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.
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 by 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.