A sudden war of words in the Shiite heartland of Iraq has highlighted fractures barely hidden beneath the surface. Shiite cleric and politician Muqtada Al-Sadr in early June launched a vicious tirade against a prominent Islamic scholar from Saudi Arabia based in Najaf. 

On the face of it, the dispute revolves around remarks made by Muneer Al-Khabbaz a decade ago—ostensibly so upsetting now to Sadr that he has suggested that the Saudi scholar should leave Iraq. Amid speculations about the underlying motives, two theories stand out. On the one hand, given Khabbaz’s close association with Grand Ayatollah Ali Al-Sistani—the supreme religious authority in Najaf—some link the clash to the looming leadership succession at the...