What will Iran do with a nuclear weapon? The many hypothetical answers to this question are behind so much of the urgency in the international situation today. Will so-called “apocalyptic Shi’ites” in the Islamic Republic seize the initiative and destroy the state of Israel? Will the Iranians take a lesson from rogue nuclear scientist A.Q. Khan and sell their technology to the highest bidder or to suicidal anti-Western proxies? Will they do none of these things, but still unbalance the fragile India-Pakistan detente enough to bring South Asia into a nuclear war? Or will the Iranian acquisition of a nuclear bomb merely spark a regional arms race, spurring undemocratic and less-than-stable regimes like Saudi Arabia, Jordan or Egypt to build nuclear warheads of their own?

All of these options would lead to a drastically more dangerous world, and they are all fairly plausible. But among the many stabs that have been taken toward evaluating Tehran’s objectives and policy, few have considered a motive that I believe to be at least as likely as any other. Iran is not building the bomb to use it. They are building it to build it.

To explain what I mean by this, you have to know something about Shi’ite Islam. In the face of the Iraq war and the growing Iranian nuclear program, Shi’ism has become an important social element of the international crisis that belongs to all of us now. This is certainly not to blame the Shi’ite faith for recent developments, but if we want to understand the impasse confronting the diplomatic community, it’s worth knowing the basics about the cultures involved.

The origins of the Sunni-Shi’ite split lie in a dispute over who should succeed Muhammad as the leader of the Islamic community. Shi’ites favored a blood relative of Muhammad as their leader, while Sunnis preferred to choose a ruler by election. In the year 680 C.E., the conflict between Shi’ite-supported Husayn and his rival Yazid met a tragic end with the slaughter of Husayn and his followers at Karbala. Husayn became the central martyr of the Shi’ite religion, and modern Shi’tes commemorate his death yearly. Sympathy for Husayn is a central feature of Shi’ism, and bitterness against Yazid has reached such extremes that, during the 1980s Iran-Iraq war, the Iranian media ceased calling Saddam Husayn by his last name, preferring instead “Saddam Yazid.”

Iraq is a historical center of Islamic tradition, especially with respect to Shi’ism, but also from the point of view of Islam in general. Not far from the original Husayn’s burial shrine in Karbala is the city of Najaf, where his father (Ali, a lawful successor of Muhammad revered by both Sunnis and Shi’ites) is enshrined. North of Baghdad, meanwhile, is Samarra, which contains the shrines of three of the 12 Imams, the historic leaders of the Shi’ite community. While these cities are unparalleled in their significance to the Shi’ite religion, the whole mass of eastern Iraq is populated with cities that have great significance for all Muslims, from Basra to Kufa to Baghdad.

Not only does Iraq contain the heartland of Shi’ite Islam, it also has ancient links with pre-Islamic Iran. The Persian Sassanid Empire was centered at Ctesiphon, near the eventual site of Baghdad. Not only does a great part of the Iraqi state have immense cultural links to the Islamic ideologues in the Iranian government, it also has some value to the considerable force of Persian nationalism.

Given all the attractions for an expansionist, Persian, Shi’ite state like Iran, is it really a far-fetched notion that the Ayatollahs are interested in Iraqi territory? Not only is there oil and manpower to be gained, but also considerable religious and symbolic capital. Are the Iranians holding out to pick up the fragments of Iraq, in terms of oil, holy sites and an enthusiastic mass of Iraqi Shi’ites? Do the Ayatollahs believe that the Shi’ite population of Iraq, persecuted and brutalized by Saddam and terrorized by Sunni militias, might be eager to align themselves formally with the Islamic regime in Tehran? Are they hoping that their fervor might be the antidote to the lagging support and religious apathy of Iran’s secularist middle class? Or are they merely hoping to build the bomb that Saddam failed to build (thanks to the Israeli Air Force) in the 1980s, in order to beat back Iraq once and for all?

In other words, when we talk about the Iranian bomb, we should stop thinking so much about Israel, the U.S., Russia, China, Korea and Pakistan. More than most have realized, the road to Tehran — literally — is through Baghdad. But because we have tossed the Iranian crisis into the proliferation bin, we have failed to recognize how much the nuclear program is a smokescreen for Iranian ambitions in and vis-a-vis Iraq.

The painful irony of this situation is that it is a side effect of “Operation Iraqi Freedom.” It is inconceivable to me that policy planners neglected the possibility of Iran being an incidental beneficiary of an American invasion of Iraq. But it seems that American diplomats once more undervalued the importance of historical enmity.

During the 1980s Iran-Iraq war, one of the major disappointments of the Ayatollahs was that Iraq’s Shi’ite population did not do more to destabilize Saddam’s secular rule during this struggle. Conversely, Saddam’s fears that this might happen were the principal reason for his savage persecution of the Shi’ite majority. This in turn is one of the reasons why inter-factional cooperation has been so difficult for Iraqis to attain. If you have trouble understanding why Iraqi Sunnis, Kurds and Shi’ites don’t trust each other, think about Vichy France, 1945, where the Nazi collaboration problem was so severe that President Roosevelt thought a civil war in France was a realistic possibility.

In contrast, the Bush team continues to insist that there will be no civil war in Iraq, even as observers on the ground are declaring that it has already begun.

Peter Durning specializes in the history of Russia and the Middle East. Write to him at pdurning@stanford.edu.