KIRKUK, Iraq — A Kurdish soldier wheeled a pickup past some buildings smashed in a recent ISIS advance and cast a wary glance at an Arab village down the road.
The frontline was a few miles away, a long canal that cuts through the oil-producing city of Kirkuk. Kurdish forces were dug in on one side and ISIS on the other. But the militants across the water weren't the only enemy the Kurds thought they faced as they braced for another assault. They saw a threat from within their own territory too — in the Sunni Arab villages they considered bastions of enemy support. "They're all the same," the soldier said. "They all support ISIS. And when they get the chance, they will shoot us in the back."
Kirkuk has been one of the toughest battlegrounds for ethnic Kurdish forces, known as the peshmerga, in their U.S.-backed war with ISIS. A mixed Kurdish and Arab city 150 miles north of Baghdad, it shares the conflict's ethnic and sectarian fault lines and underlines the difficulties the peshmerga face in Arab areas outside their comfort zone — an issue of growing importance as the U.S. promotes a plan to have its Iraqi allies launch an offensive to take the city of Mosul, deep in ISIS' Sunni Arab stronghold. "The peshmerga have never moved outside of their own territory," said Christopher Harmer of the Institute for the Study of War. "If they aren't trusting the relatively minor number of Sunni [Arab] villages they have in their sphere of influence now, how they going to roll an armed convoy into Mosul?"
The wreckage along the road in Kirkuk came from a surprise attack southwest of the city on Jan. 30, when ISIS crossed the canal under cover of fog and wreaked havoc until the peshmerga drove them back with the help of U.S. airstrikes. Kurdish soldiers said men from the handful of Arab villages in the area had joined the fight on ISIS' side. These claims couldn't be independently verified. But they showed the intense suspicion many Kurds harbor after more than six months of war. "They stabbed us in the back," said Kemal Kirkuki, who oversees peshmerga forces in the area.
Asked how many residents of the villages in question he believed to be ISIS supporters, Kirkuki replied, "most of them." He expressed deep reservations about the idea of the peshmerga pushing beyond Iraq's Kurdish region. "We might get rid of the ISIS occupation," he said. "But we'll never get rid of the ISIS mentality, because it's inside them."
Most of Iraq's Sunni Arabs do not, in fact, support ISIS. Those who resist the group have fallen victim to it, and many who live under its control feel they have no choice. ISIS has been keen to exploit Iraq's ethnic and religious divides. Sunni Arabs were targeted by the hardline sectarian policies of the Shiite-dominated government that followed the U.S. occupation, and many remain deeply suspicious of the Iraqi authorities and security forces. Since ISIS took over Mosul this summer, powerful Shiite militia working alongside the Iraqi military have been accused of carrying out executions and other human rights abuses against Sunnis.
Jordan Perry, an analyst at the Verisk Maplecroft research firm in London, called the participation of these Shiite militia — which are backed by Iran and reportedly work with Iranian military advisors on the ground — in a Mosul offensive "inevitable."
"They will play a significant role," he said. "It's a real difficulty — but that's where the manpower is."
Many Sunni Arabs are wary of the Kurdish forces too. In a report published on Thursday, Human Rights Watch accused them of destroying dozens of Sunni Arab homes in areas reclaimed from ISIS since the summer and preventing many Sunni Arab residents from returning home.
U.S. officials have said the centerpiece for the plan to retake Mosul will be the Iraqi military. The U.S. is reportedly working to ready five Iraqi brigades, which will be complemented by another three from the peshmerga as part of a mixed force numbering between 20,000 and 25,000 troops. U.S. officials have said that the offensive could begin as early as the spring. It would be the signature battle for the U.S. and its allies in their war on ISIS — challenging the group at the heart of the caliphate it declared after its shock offensive in Iraq this June.
Yet it's unclear that the Iraqi military — which was routed by ISIS in Mosul this summer and has lost control of much of Anbar province for more than a year — is ready to shoulder such a heavy load. And so far the peshmerga, as well as the Shiite militia, have been the "two most successful fighting forces against ISIS in Iraq," said Michael Knights, an Iraq expert at the Washington Institute for Near East Policy.
"The Kurds will be holding down ISIS in certain fronts and doing small advances here and there [...] and some exploiting into Mosul city to get some of the old Kurdish neighborhoods back, but not much more than that," Knights said. "What probably hasn't been factored in [to the U.S. plans] is that of the security forces in Iraq, one of them, the Kurds, is almost as far as it's ever going to go."