Two factors strongly at odds with one another in Iraq have created something of a lose-lose situation for US politicians: the sheer brutality of ISIS on one side, and a strong resistance to commit ground forces to yet another counter insurgency in the Middle East on the other. Both options are loaded. Deploy US troops in the region and antagonise a war-weary public with yet another expensive operation in Iraq, or simply rely on air strikes and risk looking ineffective, indecisive and in denial about the extent of the problem?

With these two options – to invade or not to invade – apparently mutually exclusive, the US Government has explored other strategies in the hope of finding some way to curb ISIS without using its infantry; among them, the equally controversial option of collaboration with Iran. Neither party wants to watch Iraq descend into the chaos that ISIS promises; working towards avoiding this could provide the panacea to a history of poisonous relations.

"They may not have a clear sense of the kind of Iraq they’d like to see, they may not share that vision of the country, but they share a vision of what they wish to see Iraq avoid," says Shashank Joshi, a senior research fellow for the Royal United Services Institute (RUSI).

Enemies unite

"It’s not a partnership in a very meaningful sense, in that there’s no level of detailed sharing of plans, intelligence or information that we know of. There’s no joint ability preparation or activity."

The ominous rise of ISIS has already seen something of an unofficial détente between the two traditional enemies. Although the two states would be unwilling to call any tacit cooperation a partnership – total denial is more than likely – there have been examples of closer cooperation.

The most obvious came in December last year, when Iranian and US fighter jets struck ISIS key targets in what was widely believed to be a coordinated effort, despite Tehran’s denial of the event. With the US unwilling to provide ground troops, Iran’s regular armed forces and paramilitary units could prove fundamental against ISIS in regions impermeable to airstrikes.

But despite the obvious strategic benefits of cooperation, some significant obstructions make it improbable. With both sides strangled by their domestic political scene, even talk of an alliance is rebuffed by officials on either side.

"Partnership might be putting it too strongly; I think what we’ve seen in the first instance is deconfliction," says Joshi. "In a military sense, what that means is deconfliction of air and battle space when both sides have combat aspects, whether orthodox or unorthodox, in those zones."

If both parties were to ramp up cooperation, they would need to overcome significant operational challenges as well as ideological ones.

"It’s not a partnership in a very meaningful sense, in that there’s no level of detailed sharing of plans, intelligence or information that we know of," says Joshi. "There’s no joint ability preparation or activity. At a tactical level, they don’t train together and therefore don’t know each other’s protocol. They don’t know each other’s communication frequencies or share encrypted communication.

"At the broader level, the challenges include the fact that you have very different means of accomplishing the same objective. Even if they’re aiming at the same objective, which is to weaken ISIS and shore up the Iraqi state, there are going to be fundamental differences in how you achieve that."

Unity is only skin deep

Aside from strategic differences, there are other potential barriers to a deeper partnership. The Republican-led domestic backlash is not the only disapproval the US Government is worried about. Closer ties and more coordinated attacks could be viewed as an attempt to placate Iran, alienating other more reliable partners in the Middle East that are not so keen to work with them or increase its influence in any way.

"If you have a coalition that includes Saudi Arabia, and then you also want to tacitly work with Iran, then you obviously have two partners who are at each other’s throats, so you risk upturning one set of alliances by pursuing your other partnership," explains Joshi.

"Saudi Arabia is deeply concerned about Iran’s strategic influence in the region. It’s going to be extremely concerned if it looks like you are accommodating Iranian actions or legitimating its presence in a very sensitive part of the neighbourhood. If you are working with the country that is at the same time ideologically threatening your other major oil-producing ally, you have to manage different partnerships. You can’t just pursue one without regard to the others – they often have a trade-off between themselves."

Dividing differences

Luckily for Saudi Arabia, it is unlikely that there will be any further serious cooperation between the US and Iran anytime soon, at least until the countries’ enormous differences – primarily the Iranian nuclear programme – are addressed.

"The big issue between the US and Iran continues to be getting rid of the Iranian nuclear programme," says Bruce Bennett, senior defence analyst at RAND Corporation. "That’s the impediment, but Iran doesn’t seem to want to agree to something like that, and the US is reluctant to work with a state that is pursuing a programme that it shouldn’t be."

Although talks on the nuclear programme continue, the US is keen to avoid letting any deconfliction or tacit cooperation play into negotiations and affect the framework of the agreement. There are concerns that Iran could try to leverage its position under any potential détente and secure concessions in any agreement. With American politicians already split on the treaty as it is, any further attempts by Iran to secure leniency could be a deal breaker.

"One has to talk about priorities," says Bennett. "The US does not want to see Iran, Saudi Arabia and a number of other states developing nuclear weapons and posing threats against each other. That’s a pretty high priority at this stage, probably more so than ISIS."

Although tackling the barbarity of ISIS is hugely important for the current US Government, and is likely to remain so for the foreseeable future, any further cooperation with Iran would probably require commitments on the tactics they use. There are severe tensions over the means each side uses to combat its adversary; while the US is keen to see native Sunnis turn on ISIS, the Iranian Government wants to see its own allies and properties inside Iraq flourish. Many of these proxies include groups that are antithetical to Sunnis and will ultimately inflame that particular community, perpetuating the violence.

"Whenever you get involved in a relationship with a country, you’ve got to make sure from the beginning that you set the right basis for that relationship … Iran doesn’t look like a good long-term partner." 

Another stumbling block is the fact that the US is keen to ensure it avoids getting into bed with any countries that commit war crimes or human rights violations during conflict.

"Would it follow international norms, or would it act with some degree of barbarity that is comparable to the way ISIS operates with military forces?" says Bennett. "You don’t want to be partnering with a country that’s going to break international standards and get blamed for its bad behaviour; it’s very hard to have a good relationship with a country that is violating international norms."

Reluctance about the long term

While there is closer military cooperation, any hope that this could lead to a détente in US-Iranian relations in the long term is misplaced. "The problem is that whenever you get involved in a relationship with a country, you’ve got to make sure from the beginning that you set the right basis for that relationship," says Bennett. "We are clearly concerned about ISIS, but you have to choose your partners, and it’s not clear that Iran is a desirable partner for the future. It may be a facilitator for dealing with ISIS, but it doesn’t look like a good long-term partner."

Both sides are likely to remain at best cautious of each other: the US has deep misgivings over the potentially corrosive role played by the Shia militia in Iraq, and the affect that has on the Sunni community, while Iran is concerned about the implications of its neighbour having large amounts of US firepower. But as the brutality of ISIS shows no signs of letting up, the fate of many people across the Levant rests on both sides overcoming their stark differences, albeit temporarily.

"In most cases, of course, you revert to hostility," says Joshi. "If you have a military alliance, and if that military imperative – what holds it together – is removed, then often the trust built from working together does not hold on long enough."

Shashank Joshi is a research fellow at RUSI and a doctoral student of international relations at Harvard University’s Department of Government. With master’s degrees from Cambridge and Harvard, he specialises in international security in South Asia and the Middle East. He has worked for the National Democratic Institute in Moscow, Citigroup in New York and RUSI’s Asia Programme.

Bruce Bennett has a PhD in policy analysis from Pardee RAND Graduate School and a BS in economics from the California Institute of Technology. He is a senior defence analyst at the RAND Corporation, working primarily on research topics such as strategy, force planning and counterproliferation within the RAND International Security and Defense Policy Center.