IN HOMES and coffee shops across the Gulf, reactions to the framework agreement announced last week had little to do with opinions on numbers of centrifuges. More important was the way in which a change in Iran's relations with the outside world would alter the balance of power in the Middle East. At stake, many say, is Iran’s ability to meddle in the region. Tehran already backs armed allies in Syria, Lebanon, Iraq, Yemen and (by some accounts) Bahrain. An Iran freed of pariah status might well become more aggressive. "The idea of a Shiite crescent in the region has become obsolete,” says Hassan Hassan, an Abu Dhabi-based analyst. “Today, it's a full moon and the Gulf is surrounded."

Iran's rivalry with Saudi Arabia has in recent years been soaked in a pool of sectarian bad blood. Iran “thinks that the Shiites have to conquer the Gulf,” says one tribal Sunni figure here, summing up a common perception. Meanwhile, Shias in Kuwait say they are assumed to be disloyal, simply because their sect coincides with that of the Iranian regime. Gulf officials privately say that they are watching communal relations with trepidation. In Kuwait, where between 30% to 40% of the population is believed to be Shia, Twitter is the main sectarian battleground. “We have a good standard of living in Kuwait,” one Kuwaiti man noted. “But in poorer countries, sectarian issues are really a problem.”

Regional competition between Riyadh and Tehran is nothing new; ascendent Iranian influence is, however. Sanctioned out of international energy markets, Iran’s economy limped along for much of the past decade while soaring oil prices buoyed the Saudis. Yet in the chaos unleashed by the Arab Spring both powers have worked to extend their influence—the Iranians arguably with greater success. A true rapprochement between American and Iran, however unlikely, could entirely alter the tenuous balance of power. A nuclear deal “is the worst thing that can happen,” says Khaled Sultan, a Salafist and former Kuwaiti parliamentarian. “We had hoped that the changes in Syria—success of the opposition—would deter Iran and downsize it. But [we lose hope] when we see the American government playing footsie with Iran.”