For decades the old adage the enemy of my enemy is my friend has almost always been true in the relationship between Washington and Tehran. But the rapid and brutal advent of the Islamist group ISIS in Iraq has suddenly changed the traditional strategic calculations in both the US and Iran.

Neither country has any interest in Iraq falling apart and becoming a hotbed of sectarian violence any more than it already has since the US invasion in 2003. So in that sense it was only logical that Washington and Tehran are expected to start talks about how the unlikely allies can work together to curb the spread of ISIS on Iraq.

"This is something they should have done some time ago, but it's better late than never," says Rouzbeh Parsi, an Iran expert at Lund University in Sweden. "You can see that once things come to this kind of extreme situation the ideological elements of rhetoric kind of fade away and they have to focus on the realities."

Division of labor

While neither side has stated how a possible military cooperation against ISIS could look, the blueprint for it seems clear. Essentially Washington would provide air support and Tehran would be the eyes and ears on the ground.

With President Barack Obama having excluded sending troops to Iraq, the US could launch airstrikes against ISIS through its drones and provide intelligence to Iraqi troops. Iran on the other hand could use its close ties to the various Shiite militias to provide a sense of the reality on the ground. It could also dispatch a limited number of special forces from its Revolutionary Guards unit to the neighboring country.

Compared to military cooperation, the political part of countering the ISIS extremists could prove more difficult for the US and Iran as they have to reign in Iraqi Prime Minister Nouri al-Maliki whose anti-Sunni governing style has been a great factor in sparking the current crisis.

Pushing Maliki

"This is going to be a point of contention, because the United States has not held Maliki accountable for his divisive policies in Iraq. And this has caused tension between the US and Saudi Arabia," says Lina Khatib, director of the Carnegie Middle East Center in Beirut. "If the US continues to turn a blind eye to Maliki's divisive policies then Saudi Arabia will regard any cooperation between the US and Iran to counter ISIS as playing right into the hands of Maliki politically."

Tehran, adds Parsi, also has "invested a lot in the Maliki government and this whole set-up of Iraqi politics." To achieve anything resembling a stable and sustainable Iraq, the US and Iran must push Maliki to change his anti-Sunni stance. In addition, Saudi Arabia who has been fighting a proxy war with Iran in the region including Iraq for years must be brought in to play a constructive role.

"So in the long run you need domestically in Iraq get the Shia dominated government to bring in the Sunnis, but you also need the Saudis to help as well by telling the Sunnis we are not going to finance militias on your part to wage war inside Iraq," says Parsi.

New page

Both experts are convinced that the US and Iran are indispensable to halt ISIS. "But to be honest, I am not sure that even with US and Iranian cooperation we will necessarily see a defeat of ISIS as such", notes Khatib. "However, any improvement in the relationship between Iran and the United States can only be a good thing for stability in the Middle East. And I very much hope that the current crisis can push the two sides to political compromise that would then give groups like ISIS less of a raison d'etre to continue to expand in the region."

But regardless of the eventual military and political outcome in Iraq, argues Khatib, the current crisis provides a unique chance for Washington and Tehran to make another bold step in revamping their ties with consequences far beyond the immediate region:

"In the rise of ISIS we see an actual golden opportunity for the United States and Iran to start a new page. The ISIS takeover of Mosul can be seen as a motivator for Iran and the US to engage in a more open relationship and to push the negotiations between the two countries forward."