3 worse if tensions continue to ratchet upward. They see a heightened possibility of actual military conflict in the Gulf, where one-fifth of the world's oil supplies traverse the shipping lanes between Saudi Arabia and Iran. Growing hostility between the two countries could make it more difficult for the U.S. to exit smoothly from Iraq this year, as planned. And, perhaps most dire, it could exacerbate what many fear is a looming nuclear arms race in the region. Iran has long pursued a nuclear program that it insists is solely for the peaceful purpose of generating power, but which the U.S. and Saudi Arabia believe is really aimed at producing a nuclear weapon. At a recent security conference, Prince Turki al Faisal, a former head of the Saudi intelligence service and ambassador to the U.K. and the U.S., pointedly suggested that if Iran were to develop a weapon, Saudi Arabia might well feel pressure to develop one of its own. The Saudis currently rely on the U.S. nuclear umbrella and on antimissile defense systems deployed throughout the Persian Gulf region. The defense systems are intended to intercept Iranian ballistic missiles that could be used to deliver nuclear warheads. Yet even Saudis who virulently hate Iran have a hard time believing that the Islamic Republic would launch a nuclear attack against the birthplace of their prophet and their religion. The Iranian leadership says it has renounced the use of nuclear weapons. How a string of hopeful popular protests has brought about a showdown of regional superpowers is a tale as convoluted as the alliances and history of the region. It shows how easily the old Middle East, marked by sectarian divides and ingrained rivalries, can re-emerge and stop change in its tracks. There has long been bad blood between the Saudis and Iran. Saudi Arabia is a Sunni Muslim kingdom of ethnic Arabs, Iran a Shiite Islamic republic populated by ethnic Persians. Shiites first broke with Sunnis over the line of succession after the