By Michael Singh –  Wall Street Journal –  June 17, 2014 – ISIS would likely welcome deeper involvement from the IRGC.

During the second Iraq war, Iran’s Islamic Revolutionary Guard Corps was one of the United States’ fiercest foes. The IRGC was responsible not only for organizing, training and equipping Shiite militants who fought U.S. troops, but also for manufacturing and importing into Iraq so-called explosively formed penetrators, or EFPs, one of the chief banes of American forces there. Also courtesy of Tehran: mortar and rocket attacks on the Green Zone in Baghdad, designed to speed the American departure.

For this reason, in addition to the deep distrust that has characterized U.S.-Iran relations since 1979, it is more than passing strange to hear both American and Iranian officials mooting the possibility of U.S.-Iran cooperation in Iraq today. The U.S. and Iran share an interest in preventing further advances by the extremist Sunni militia that calls itself the Islamic State in Iraq and al-Sham, or ISIS. Nevertheless, accepting Iran’s offer of assistance in Iraq would be a grave mistake.

The efficacy of Iranian assistance is dubious at best: The IRGC has proven adept at stoking insurgencies, but no better than any other country at ending them, in Syria or in Iran itself. Not only would a partnership between Washington and Tehran likely fail to improve the situation — it could make matters far worse in Iraq and throughout the Middle East. The current crisis in Iraq is not precisely a sectarian conflict. ISIS hardly enjoys unanimous support among Sunnis, who are among the group’s victims as it imposes repressive rule in the areas it conquers. But sectarian tensions are an important factor in the country’s problems. Iranian (Shia) involvement would be perceived by Iraqi Sunnis as explicitly sectarian in nature, and thus do more to inflame those tensions than calm them. For their part, Iraqi officials largely appear to recognize this danger and have thus been publicly wary of embracing Iranian offers.

In contrast, ISIS and other radical groups would likely welcome deeper Iranian involvement. ISIS seeks to stoke anti-Shia sentiment to garner both local and outside support. Were Iran to become more directly involved in Iraqi affairs — especially in concert with the U.S. — ISIS would take it as a propaganda boon and use the development to attract funding and fighters.

This polarizing effect would be magnified if Iran resorted to organizing and equipping Shiite militants. These militants might help halt ISIS advances in the short run, but their reactivation would threaten to return Iraq to the days of open sectarian war. Because they are an alternative to a professional fighting force, these militias also pose an institutional threat to efforts to cultivate a cross-sectarian Iraqi army.

Deeper IRGC involvement would increase Iraqi Prime Minister Nouri al-Maliki’s dependence on Iran. His success thus far has been in part due to the perception that his rivals in Iraq’s Shiite community — which is far from monolithic — were too close to Tehran. Turning now to Iran for aid could change that, and not by accident; while the U.S. seeks an independent, pluralistic Iraq, Iran appears to prefer that Iraqi political and clerical institutions be beholden to Tehran’s own.

Iranian intervention in Iraq, whatever its immediate tactical utility, would deepen the country’s cleavages. And U.S.-Iranian cooperation in Iraq would stand at stark odds with President Obama’s sensible call for outreach by Mr. al-Maliki to Iraq’s Sunnis.

It could also have repercussions beyond Iraq. As the United States has stepped back from its traditional security role in the Mideast, a contest by proxy has emerged among regional powers such as Iran, Saudi Arabia and Turkey. The front line of this conflict has been Syria. It has now expanded to Lebanon and Iraq, and may expand elsewhere.

America’s allies have worried that we are at best neutral in this conflict, and at worst willing to overlook Iran’s regional activities to expedite a nuclear agreement and accelerate a “pivot” away from the region. Some even worry that the U.S. is seeking a new alliance with Iran to supplant its old alliance system in the region. As misplaced as these worries may be, an American embrace of an Iranian security role in Iraq — or even bilateral talks with Iran on regional security that exclude other stakeholders — will exacerbate them.

The U.S. goal must not only be to drive ISIS back from Baghdad, but also to organize allies in an effort to halt the spread of chaotic regional conflict, and to restore some semblance of stability and optimism. For the U.S., this need not involve boots on the ground, but it will require diplomatic re-engagement and a willingness to employ force judiciously, where appropriate. What is needed from Iran, meanwhile, is not more involvement in regional conflicts, but less. Specifically, Tehran must end its support for the Assad regime in Syria; its provision of arms, funding and equipment to Sunni and Shia extremist groups alike (such as Hezbollah and Hamas); and its nuclear brinkmanship. This would do far more to improve prospects for the Middle East than the deployment of IRGC irregulars in Iraq.

Michael Singh is managing director of The Washington Institute.