A powerful Iranian general has emerged as the chief tactician in Iraq’s fight against Sunni militants, working on the front lines alongside 120 advisers from his country’s Revolutionary Guard to direct Shiite militiamen and government forces in the smallest details of battle, militia commanders and government officials say.

The startlingly hands-on role of Iranian Gen. Ghasem Soleimani points to the extent of the Shiite-led Iraqi government’s reliance on its ally Tehran. It also strikes a strong contrast with the more methodical, cautious approach of the United States, Iran’s rival for influence in Iraq. Shiite fighters have come to idolize the Iranians who have moved into the heat of battle alongside them — with two Iranian advisers killed in fighting — while government officials grumble the United States has failed to come to their aid.

The Iranian role, however, risks further sharpening the sectarian rifts in the conflict. At a time when the U.S. and others are pressing Iraq’s government to reach out to Sunnis to reduce support for the insurgency, the effective Iranian command of Iraq’s defense is likely to further alienate Sunnis, who have long accused Shiite-led Iran of trying to dominate Iraq through its allies here. […]