Barack Obama has called for a "broad-based international coalition" to "degrade and destroy" Islamic State (ISIS) after the beheading of the American journalist Steven Sotloff.
But it is not clear which countries would take part in such a grouping and, crucially, whether its mission would be limited to Iraq or include fighting the jihadis in their Syrian strongholds.
In Washington and London, government officials say they had long known that their nationals were being held hostage by the extremist group, so the latest killing, plus the now explicit threat to murder a UK captive, will not change their fundamental calculations.
Talk of building a coalition to tackle ISIS has been in the diplomatic air for the past two weeks, but Obama gave deeper insight into his thinking on Wednesday: "The question is going to be making sure we have the right strategy but also making sure that we have got the international will to do it," the president said. "What we have got to make sure is that we are organizing the Arab world, the Middle East, the Muslim world, along with the international community to isolate this cancer."
UK participation in air strikes looks closer than before given David Cameron's comments about working with the Iraqi government as well as "allies and neighbours"— echoing Obama's language and assumptions.
Britain, France, Germany and Italy have all been arming Kurdish forces fighting ISIS. Outside NATO, Australia has been keen. With formal requests from the Kurdish regional government in Erbil as well as the Iraqi central government in Baghdad, a new "coalition of the willing" looks likely to come together.
Turkey, a NATO member with a powerful air force, is an obvious candidate. Iran, which is opposed to ISIS and backs the new Baghdad government but is wary of western intervention — especially against its close ally Syria – is already active.
Iraqi militias it backs helped break the ISIS siege of Amerli near Tikrit. Qassem Suleimani, commander of the Iranian Revolutionary Guards al-Quds Force, was photographed there — a very public signal. Iran could also share intelligence with the U.S., as it did in the fight against the Taliban in Afghanistan in 2001.
Arab countries are a different matter. Saudi Arabia, Qatar and the UAE — which promoted Islamist groups fighting Bashar al-Assad — have cracked down on the movement of men and money to Syria and Iraq recently. Together they command extensive air power, but it may not be used.
"It is hard for me to see how they could contribute in a meaningful way as a military coalition," said Fred Wehrey of the Carnegie Endowment for International Peace. Money and security coordination might be the limit of their input.
The Gulf states also mistrust Obama after last year's reversal on punitive air strikes against Assad in the wake of the chemical warfare attacks on rebel-held areas near Damascus. Other constraints include angering ISIS sympathizers at home and the fear that their old enemy — and Shia sectarian rival — Iran will benefit.
The Arab exception, diplomats say, may be Jordan, which has borders with Iraq and Syria and whose advanced intelligence and special forces capabilities could discreetly support U.S.-led efforts.
King Abdullah II is attending the NATO summit while Arab foreign ministers are to convene in Cairo for an emergency meeting at the weekend, where the emphasis is likely to be on preserving the territorial integrity of Iraq. Their deliberations will be closely watched for any sign of active involvement in the fight against ISIS.
Arab countries participated in the anti-Saddam coalitions in 1991 and 2003, though the region was far less divided than it is now in the wake of the 2011 uprisings.
Forming a coalition will be challenging, while operational considerations must not be subordinate to political ones, Emile Hokayem of the International Institute for Strategic Studies told the Guardian: "A coalition in which sectarian Iraqi Shia militias play a key role because these are Baghdad's only or most reliable troops, or in which Kurdish fighters are asked to operate far from their territories, could antagonize the very constituency whose support against ISIS is fundamental: the various local Sunni communities who have accommodated or been subdued by ISIS."
The looming U.S.-led intervention against ISIS does not mean that fundamental problems are being tackled. "Everyone who hated [George W] Bush's 'war on terror' — seeing it either as inadvertently pouring oil on the flames, or as an aberrant throwback to the logic of imperialism — is now happily singing from that very hymn sheet because it saves them having to think about the real challenges the region poses,"argued Peter Harling of the International Crisis Group.
Syria is still the toughest question. The Turks, Saudis and Qataris are likely to demand support for Assad's overthrow — which the U.S. looks unlikely to give. The Baghdad government, still close to Tehran, would also oppose that.
Conversely, there is little chance that Obama will respond to signals from Assad offering his cooperation in the fight against terrorism: that would involve an unacceptable volte-face as well as alienate the Sunnis, who need to be encouraged to combat the jihadis. The Syrian military, in addition, has little to offer.
But it is clear that tackling the crisis in Iraq cannot ignore the unending and de-stabilizing war next door.
"ISIS uses Syria as its strategic depth and moves fighters and assets across the now-erased Syrian-Iraqi border," said Hokayem. "A strategy to contain and roll back ISIS in Iraq would have the net effect of increasing violence and tensions inside Syria without decisively weakening the movement. The world, and the U.S. in particular, have neglected Syria for too long. Doing so now is tantamount to a strategic mistake of an even greater magnitude."
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