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. Both Saudi Arabia and Iran are theocracies, with their respective theologies locked in an irresolvable conflict that began with the death of the Prophet of Islam some 15 centuries ago. Saudi Arabia is Custodian of the Two Holy Mosques and the birthplace of Islam. It is the leader of the Sunni world, culturally conservative, and Arab.