There's an interesting article in the New Republic called, "Bombs Away: The real reason why Russia and China aren’t interested in stopping Iran’s nuclear program." The author, Matthew Kroenig, rejects the common explanation that Russian and Chinese economic ties with Iran are the primary reason for their assistance with its nuclear aspirations, and for their reluctance to do anything about it. Instead he looks toward strategic reasons. He argues that both countries see a nuclear Iran as a constraint on U.S. power in the Middle East -- a constraint that will not affect Russian & China because they lack force projection capabilities in the region.
An Iranian bomb, then, won’t disadvantage China or Russia. In fact, it might even help them. Neither country has hidden its desire to hem in America’s unilateral ability to project power, and a nuclear-armed Iran would certainly mean a more constrained U.S. military in the Middle East. Indeed, at times during the 1980s and 1990s, Beijing and Moscow aided Tehran with important aspects of its nuclear program.It's nice to see an argument on this issue that gets away from economic determinism and looks toward power politics and grand strategy. I think the author is probably correct in his view of Russian & Chinese motives. Where I disagree with him is on his analysis of the U.S. side of the equation. Here he offers a reverse of Chinese & Russian motivations. The U.S. opposes proliferation in Iran because it constrains U.S. power projection in the region.
Of course, there are the concerns of accidental nuclear detonation, nuclear terrorism, or even nuclear war. But these are all extremely low probability events. The primary threat of nuclear proliferation is that it constrains the freedom of powerful states to use or threaten to use force abroad. ... In nearly every historical instance of proliferation, beginning with China in the 1960s, the United States opposed nuclear proliferation in large part because it wanted to preserve its military freedom of action. ... Although Washington might not have immediate plans to use force in the Middle East, it would like to keep the option open.Although I agree that this reasoning is certainly part of the U.S. desire to block Iranian nuclear weapons, I think he ignores other obvious reasons of greater significance. First and foremost, Iran is a rogue state ruled by radical Islamic fanatics hostile to the U.S., and has been conducting a low level proxy war against U.S. forces in the region. It's not really necessary to look for deep strategic reasons to understand why the U.S. would want to keep nuclear weapons out of the hands of the mullahs. If Iran was ruled by a more responsible government that just happened to be aligned against the U.S., the author's argument would be more convincing.
Second, Kroenig ignores the threat to Israel in his calculations. This is a key reason the U.S. opposes a nuclear weapon in Iranian hands. Israel rightly sees such weapons as an existential threat, and the U.S. would rather not be sucked into a Middle East war if the Israelis decide they must act to preempt the threat. And third, there is the fear that even if war doesn't break out right way, that an Iranian bomb will precipitate a Middle Eastern nuclear arms race, with other countries feeling that they need to obtain weapons, leading to a general destabilization of the region with unknown consequences.
In my opinion, the nature of the Iranian regime, its hostility toward the U.S., the threat to Israel, the fear of being drawn into a war, and worries about the stability of the region as a whole are far more significant motivations for U.S. opposition to Iranian nuclear arms than our desire to maintain future force projection capabilities in the region.