Newsweek's Fareed Zakaria offers a subtle, cogent case for why the United States should not fear a nuclear Iran. He makes two arguments: first, as the example of China shows, messianic states that acquire nuclear weapons do not always fulfill their genocidal rhetoric, and second, that Arab countries will continue to balance against a rising Iranian power.
Both of these are carefully made arguments. Unfortunately, neither are particularly reassuring, or persuasive.
I have never been too enamored with historical analogies, and Zakaria fails to make the case as to why Iran will become pacified like China, but his subtle point is a good one -- that countries that adopt the most alarmist interpretations of their enemies' conduct tend to implement bad foreign policy.
Zakaria's second argument is more underwhelming. He writes that, "Arab regimes will get more assertive in responding to the rise of Iranian power." What does he mean by "more assertive"? Egypt is musing about developing a nuclear capability of its own – is that what he means by "assertive"? Also, among Sunni states, Egypt is probably the least worried about the rise of a Shia power. How will other Middle East states react?
Also, since the United States wields more influence over most Arab states than it does over Iran, should it encourage the nuclearization of the region in response to an Iranian threat? And how would more militarily assertive Arab states affect the Arab-Israeli imbroglio? And might this ossify political reform in the Middle East, with Arab states and Iran using their possession of nuclear technology to deter U.S. support for democratic reforms?
Ultimately, those who seek to make more palatable the idea of a nuclear Iran need to reckon with the massive imbalances this will cause both to the Middle East, the United States' ability to be the dominant actor in the region, and the future of Middle East political reform.