Last week's advance by Sunni insurgents in Iraq provides a powerful argument for why Iran and Saudi Arabia should bury their Cold War-style feud, but is nonetheless likely to set back detente between the Persian Gulf's dominant Sunni and Shiite powers.
After decades of often overt Saudi-Iranian hostility that polarized the Middle East — and three years of proxy war in Syria — the Sunni monarchy and Shiite revolutionary state had gingerly begun in recent months to explore ways to reach out.
Saudi Arabia announced in May it had invited Iranian Foreign Minister Mohammed Javad Zarif to make a rare visit. Earlier this month, the emir of Kuwait — a Sunni monarch and close Saudi ally — made the first visit to Iran by a Kuwaiti head of state since the 1979 Iranian Revolution. He met Supreme Leader Ali Khamenei.