In the last few years, the United States has resolved to reduce its “footprint” in the Middle East, and has conceived of détente with Iran as the means of accomplishing this.
If the Islamic Republic could be drawn into the regional security architecture, forge a new relationship with the Sunni-led neighbouring states that creates a self-sustaining balance of power, and be deputised to lead on issues of common interest—such as the fight against the Islamic State—it would require less U.S. involvement to police the area.
Unfortunately, Tehran does not see the U.S. withdrawal as a chance for co-operation and integration into the present state system; it sees it as a chance to overturn that state system and replace it with its own hegemony. With Russian assistance, an imperial space under the domination of Iranian ground forces has now been extended through contiguous areas of Iraq, Syria, and Lebanon, and the process of exporting the Islamic revolution is underway in other areas like Yemen and Afghanistan.
Iran cannot defeat Sunni militancy because its own sectarian structures provoke more antagonists than they quell. Iran also has minimal interest in defeating IS, at least right now: IS is a useful foil for the extension of Iranian power. Moreover, Iran is a threat—and a more sophisticated and durable one—than any Sunni jihadist group.