At best, the governments in Washington and Tehran don't fully trust each other. More often, they fear and loathe each other. Each government has a record of abusing the interests and the people of the other. The CIA overthrew the Iranian government in 1953; the Iranians took dozens of Americans hostage in 1979. The two countries' current leaders have struggled, and so far failed, to reach a lasting agreement about international control of Iran's nuclear program.
Yet everyday brings more signs from both sides, that the two countries are aligning their armed forces and policies towards degrading and destroying the embryonic Islamic State that has emerged in eastern Syria and Western Iran.
The latest signal: The January 8 comment of Gen. Martin Dempsey, chairman of the US Joint Chiefs of Staff, that Iran's influence in Iraq could be "positive." Dempsey was more guarded than effusive but in a situation where the U.S. needs all the help it can get, America's top general isn't slamming the door shut.
Nor are the Iranians. Last week, former Iranian nuclear negotiator Hossain Mousavian called for "quiet collaboration" on fighting extremism and terrorism in the region. "Such actual cooperation, preferably without fanfare," he wrote, "carries the potential for further expansion to include other regional hotspots and crises. It's not an empty claim: Iran has been — and will be — an anchor of regional stability."
That's the key: "without fanfare." The Iran and the United States are so alienated that even to speak of cooperation will be problematic in both capitals. Yet on the ground, where the Islamic State is beheading Americans and massacring Shi'a, the two governments have the exact same interest: crush the Sunni Salafists before they destabilize the region even more.
And there is a natural division of labor. The Iranians put boots on the ground while the Americans put the planes in the air.
Iran has good relations with the Shiite militias that have replaced the collapsed Iraqi army on front lines. The Iranian leaders are frank about their desire to "annihilate" the Islamic State, They have already supplied weapons to the Kurds, and they have been taking casualties since June.
The Americans can bomb the Islamic state without boots on the ground (which a war-weary public and Congress do not want). The aerial attacks seemed to have at least checked the Islamic State's expansion.
Of course, suspicions are high on both sides. An analyst for Iran's Fars news agency argues the United States is not serious about attacking the Islamic State. Over here, a former U.S. general sees Iran as a "dangerous ally."
Yet on the ground, their interests are virtually identical. Cooperation will have its costs for both capitals. But so will non-cooperation. And that is new for the U.S. and Iran.