The US air campaign to thwart the advance of fighters from the Islamic State has been the easy part of President Barack Obama's strategy in Iraq and Syria. Soon begins the next and much harder phase: rolling back their gains in Mosul, Fallujah and other populated areas, which will require US advisers to train and co-ordinate air strikes with Iraqi forces.

Pentagon officials are more willing than their counterparts at the White House to acknowledge that this will almost certainly require US special operations forces on the ground to call in air strikes and provide tactical advice to Iraqi troops.

Gen Martin E Dempsey, chairman of the joint chiefs of staff, this week described this phase as "extraordinarily complex". Urban warfare in Iraq has been challenging for the US, which had 70 troops killed in the second battle of Fallujah in 2004 and fought hard to regain control of cities like Mosul, Baqouba and Baghdad. So it will be even harder for the Iraqis, who have proved ineffective in combating the IS.

Military officials say they plan to use Iraqi security forces, Kurdish peshmerga fighters and local Sunnis – whom they hope to turn against the militants – to roll back the IS’s gain. They see the Sunnis as playing a similar role to what played out in the Sunni awakening during the surge in Iraq.

Military loyalty

Assembling those ground forces will take time. Dempsey said that, of the 50 Iraqi brigades whose readiness the US had closely examined, 26 “were assessed to be reputable partners” with adequate equipment and leadership, to be loyal to the government and not overly sectarian.

But many of the Iraqi units will require training and re-equipping before they are ready to begin a major counteroffensive. The US is trying to institutionalise the Sunni tribal awakening by establishing new national guard units that it would have a crucial role in training and equipping. The idea is avoid the need to send a largely Shia army to Sunni areas and to win the allegiance of local Sunnis. In trying to seize urban areas from the IS's control, the Iraqis' firepower will be limited. On Saturday, Iraq's new prime minister, Haider al-Abadi, said the Iraqi military would not use artillery or carry out air strikes in populated areas.

A senior state department official said last weekend the Iraqi air force’s “targeting is not nearly as precise as ours, and they’ve made some real mistakes”. “So, that’s why Prime Minister Abadi yesterday announced that even in populated areas in which ISIL has control, we are not going to do air strikes or artillery-type stuff because it could harm the civilians,” the official said, using an another name for IS.

So it falls to the US and other allied nations to conduct the air strikes, and they will need to be carefully co-ordinated. In the last week, the offensive strikes Obama promised have started slowly, targeting a few scattered Sunni militant positions.

Close co-operation

US military advisers are already working closely with Iraqi battalions in the field and have not limited themselves to staying in Iraqi brigade headquarters, US officials said. But so far none have been used to call in air strikes. The operation to take back the Mosul Dam, in which fewer than 200 Iraqi Counterterrorism Service commandos played the critical role, along with peshmerga fighters, posed a particular challenge. Gen Lloyd J Austin III, the head of central command, had recommended deploying US military advisers to co-ordinate air strikes in support of Iraqi and Kurdish forces who had never worked together before and spoke different languages.

Given Obama’s reluctance to put US advisers alongside Iraqi combat troops, a workaround was arranged, Dempsey noted on Tuesday in testimony at a Senate hearing, in which the peshmerga passed targeting information on IS positions to an operations centre in Irbil manned by Iraqi and US troops. They, in turn, passed the information to US aircraft.

But this arrangement, as Dempsey signalled, is unlikely to be sufficient for the next phase. Austin said air controllers would be needed. “He shares my view that there will be circumstances when we think that’ll be necessary, but we haven’t encountered one yet,” Dempsey said of Austin. But the White House made clear requests to use the advisers to call in air strikes to provide tactical advice on the battlefield to Iraqi units would need to be approved.

In weighing such requests, the White House may have to choose between the increased risk to US personnel and the danger that without the use of advisers on the battlefield the counteroffensive may stall.The Iraq war provided a telling example of what can happen when the Iraqis operate largely on their own. In March 2008, Nouri al-Maliki decided on his own to mount an operation to retake Basra.

Bloody stalemate

The Iraqi military and the Shia militias fought to a bloody stalemate until the US dispatched FA18 jets, AC130 gunships and Predator drones. To help the Iraqi’s Basra campaign, US commanders also arranged for three rifle platoons from the 82nd Airborne to partner with Iraqi battalions so they could call in air strikes and back up the Iraqis. An Iraqi battalion sent deployed with its marines advisers and also had success.

Even with American help, the counteroffensive against the IS may confront an enemy rapidly adapting to the US air strikes by hiding equipment and troops under trees and tarps, and eschewing many electronic communications.

Attacking forces of the IS in Syria will come later, but first the US will have to train the Syrian rebels who will fight the militants on the ground. Dempsey said this week Pentagon planners estimated it would take eight to 12 months to train the first 5,400 soldiers; the goal is to train about 5,000 a year, Pentagon officials said. But those numbers would only be the beginning of the forces the Pentagon believes will be necessary. Dempsey said American planners estimated that 12,000 personnel would be needed to control liberated areas in Syria and restore the border with Iraq.

The CIA recently estimated the IS had 20,000-31,500 fighters, two-thirds based in Syria.

David Shedd, acting director of the Defence Intelligence Agency it was “very difficult to measure the size and capability of the truly committed”. But some seasoned military officials have questioned whether the strategy that Obama and his advisers have developed will be sufficient. “Unfortunately, the strategy in many ways will be made up on the fly.”