Pervez Hoodbhoy in Bulletin of the Atomic Scientists:
The Islamic Republic of Iran stands at the threshold to the bomb. In 2010 it had more than enough low-enriched uranium (some 2,152 kilograms) to make its first bomb's worth of weapons-grade uranium. The LEU would have become highly enriched uranium in roughly 10 weeks had it been fed into the 4,186 centrifuges then operating. Thousands of other centrifuges are also known to be operating at the Natanz secret nuclear facility. Even if Iran had not received a bomb design from the so-called father of Pakistan's nuclear weapons program, A. Q. Khan, the six-decade-old physics of implosion devices would be no mystery to Tehran's sophisticated nuclear scientists. Iran now awaits only a political decision to make the bomb.
What if Iran chooses to cross the threshold? Among other likely consequences, an Iranian bomb would be a powerful stimulus pushing the Kingdom of Saudi Arabia to follow and seek the first Sunni bomb. The first, yes. Though also a Sunni-majority state, Pakistan built its bomb not for Islamic reasons, but to counter India's nuclear arsenal. In fact, Shiite-majority Iran enthusiastically hailed Pakistan's 1998 test of an atomic device. Clearly, the Iranian leadership did not see Pakistan's bomb as a threat.
But Sunni Saudi Arabia sees Shia Iran as its primary enemy. The two are bitter rivals that, post-Iranian revolution, have vied for influence in the Muslim world. Saudi Arabia has the world's largest petroleum reserves, Iran the second. Saudi Arabia is the biggest buyer of advanced US weapons and is run by expatriates. It is America's golden goose, protected by US military might. But fiercely nationalist Iran expelled US oil companies after the revolution and is building its own scientific base.