U.S. COMMANDERS are taking an upbeat view of Iran’s close involvement in an assault by Iraqi forces on the city of Tikrit, which has been held by the Islamic State since summer. After reporting that two-thirds of the attackers were from Shiite militias and the operation had “overt . . . Iranian support,” Joint Chiefs Chairman Gen. Martin Dempsey said in a congressional hearing Tuesday that “if they perform in a credible way . . . then it will in the main have been a positive thing.”

Such optimism seems shortsighted. While any reduction in the Islamic State has benefits, the Tikrit operation raises multiple red flags. The United States was excluded by the Iraqi government of Haider al-Abadi; meanwhile, Iran has dispatched its own ground forces, artillery and drones. The assistance is being overseen by a notorious general, Qassem Suleimani, who previously supervised attacks on U.S. forces in Iraq.

Tikrit, the home town of Saddam Hussein, is part of Iraq’s Sunni heartland, so the heavy involvement of Shiite Iran and the militias allied with it could turn what is supposed to be a counterterrorism campaign into a sectarian bloodbath. Even if it does not, a victory would advance Tehran’s goal of extending its influence across Iraq, rather than being limited to the central government in Baghdad and Shiite-populated areas.

Mr. Abadi, who took office promising reconciliation with Sunni leaders, is saying that Tikrit will be turned over to Sunni police and tribes when it is recaptured and its refu­gee population invited to return. But fewer than 1,000 Sunni fighters are included in the 30,000-strong attacking force; the government has not delivered sufficient arms to Sunni tribes willing to fight the Islamic State. Moreover, Shiite militia leaders, as The Post’s Erin Cunningham reported, have portrayed the offensive as revenge for a massacre of mostly Shiite Iraqi soldiers by the Islamic State in June. For his part, Mr. Abadi alarmed human rights monitors by declaring that “there is no neutral party” in Tikrit and that residents not siding with the attackers would be considered supporters of the Islamic State.

The Tikrit operation underlines the Obama administration’s ill-advised dependence on Iran in an under-resourced Iraq strategy. According to the Wall Street Journal, U.S. officials say a tacit division of labor has developed between Iranian and U.S. forces, with American commanders focused on a planned offensive to retake Mosul, the largest city under Islamic State control, this year. But Iraqi officials are describing Iran as more committed than the United States and expressing irritation at the Obama administration’s slowness to provide resources. They have a point: Iran has deployed the front-line tactical advisers that President Obama has refused to authorize.

By allowing Iran to take the military lead in Tikrit and other parts of Iraq, the United States might speed the destruction of the Islamic State. But the administration is also risking the undoing of all the work that has been done since last summer to prevent Iraq from fragmenting along sectarian lines — and it is allowing Iran to take another step toward replacing the terrorist regime with its own malevolent hegemony.