Volunteers of the Iraqi Shiite Badr Army militia patrol an area of eastern Tikrit city, in northern Iraq, on Feb. 7. (Ali Mohammed/European Pressphoto Agency)

AN OFFENSIVE to drive the Islamic State out of the Iraqi city of Tikrit, nominally ordered by the Iraq government but spearheaded by Iranian-backed Shiite militias, has been stalled for a week, with the attackers reportedly suffering heavy casualties. For those who hope Iraq still can be reconstructed under a unified and non-sectarian government, that is half-bad news.

The resilience of the Islamic State’s defenses in the hometown of Saddam Hussein suggests that liberating larger cities across northern and western Iraq, including Mosul, might be tougher than some Iraqi officials have anticipated. On the other hand, the checking of the Shiite militias in their attempt to overrun a Sunni territory might ultimately have some positive results. The growing power of the militias, with their brutal tactics, sectarian ideology and allegiance to Iran’s most militant faction, has become as large an impediment to the goal of stabilizing Iraq as the Islamic State.

The Obama administration, focused on completing a nuclear deal with Iran and eager to minimize direct U.S. involvement in the latest Iraq war, has played down the militia menace. While not supporting the attack on Tikrit with airstrikes, senior officials have characterized it as a positive development. Such statements suppose that a force including commanders and units on the State Department’s global terrorism list and steered by an Iranian general who previously directed attacks on U.S. troops will somehow advance the aim of reconstructing a multiethnic Iraq.

In fact, a new report from Human Rights Watch documents how Shiite militias have pursued a brutal scorched-earth policy in areas already liberated from the Islamic State. After U.S. airstrikes drove Islamic State forces out of the town of Amerli, in northeastern Iraq, late last summer, the militias went on a sectarian rampage, burning and bulldozing thousands of homes and other buildings in dozens of Sunni villages. The intent was to violently alter the demography of once ethnically diverse areas so that Shiites could dominate them.

Human Rights Watch used satellite photographs to identify 3,800 destroyed buildings in 30 towns and villages in the area around Amerli. Now, studying aerial photographs of hamlets captured by Shiite forces around Tikrit, researchers say they are concerned that similar tactics are being pursued, despite the claims of the Iraqi government that the Sunni population will be protected and refugees allowed to return.

Last year, the Obama administration insisted on the replacement of Iraqi Prime Minister Nouri al-Maliki because of his Shiite sectarian agenda. His Shiite replacement, Haider al-Abadi, appears more committed to reconciling with Sunnis and Kurds. But Mr. Abadi, who has complained bitterly about the slowness of the United States to provide weapons and other support to the Iraqi Army, appears to lack the clout to follow through on his promises to rein in the Shiite militias. The Tikrit offensive is their initiative, and that of Iranian Revolutionary Guard Maj. Gen. Qasem Soleimani.

If the assault fails, those in Iraq who support Mr. Abadi’s agenda, including Kurds and Sunni tribal leaders, may have more opportunity to assert themselves. But there will be no chance of checking the Shiite militants unless the Obama administration reverses its policy of passively allowing their rampages. Instead, Washington must accelerate training and equipment for regular army units, especially in the Kurdish region, and insist that Mr. Abadi begin implementing promised political reforms.