A few months ago, intelligence officers and local officials started noticing an increasing stream of ISIS commanders and fighters leaving Mosul and heading into the Hamrin Mountains, Reuters reported on Thursday.
The Hamrin Mountains run from the border with Iran, along the southern edge of Kirkuk province, and northwest to the Tigris River. They offer militants hard to locate hideouts as well as easy access to four of Iraq’s provinces.
Lahur Talabany, director of Kurdistan’s counter-terror body, told Reuters in February that he had no doubts Mosul would be retaken, but cautioned against what will be next.
“Mosul will get taken. I think it is the asymmetric warfare that we need to be worried about,” he said.
Talabany said there are signs that ISIS fighters are sheltering in the Hamrin Mountains between Tikrit and Hawija.
"It is a very tough terrain. It is very difficult for the Iraqi military to control," he said. "It’s a good hideout place and a place they could have access from province to province without getting detected."
Video: Kurdish Peshmerga know where ISIS will go next
Although some of the fleeing militants were intercepted and caught, several managed to evade Iraqi security forces and began setting up bases to begin their new operations.
"Our jobs will become much more difficult. The army will take a rest a little, but it will be the job of security forces that will become more difficult," Talabany had warned.
Security and intelligence officials are already bracing for a new kind of destructive insurgency, such as the one al Qaeda waged in 2003 following the US-led invasion. Many ISIS fighters and leaders have roots in al Qaeda.
"They are digging in. They have easy access to the capital," Talabany told Reuters. “I believe we have tougher days coming.”
ISIS’ ultra-hardline ideology and brutality far surpassed al Qaeda’s as they took control of Mosul and seized a swathe of territory across Iraq and Syria.
Former Saddam Hussein intelligence officers also joined forces with ISIS.
“These shrewd military strategists and his Baath Party are expected to be the new generation of ISIS leaders,” Talabany said.
Iraqi and Kurdish security officials believe that after the fall of the caliphate, ISIS will put their focus on far less predictable guerrilla warfare.
"We know some of these guys escaped. They are trying to send people out for the next phase, post-Mosul, to go into hiding and sleeper cells," said Talabany.
The main concern of intelligence is if the Iraqi army will be more comfortable with conventional warfare or if they have the capability to take on an insurgency of sleeper cells.
"You have to try and find them when they go underground, you have to try and flush out these sleeper cells. There will be unrest in this region for the next few years, definitely," said Talabany.
Sleeper cells are a major concern of authorities in Kirkuk. This week, a number of alleged ISIS militants accused of being members of a sleeper cell were arrested in the latest investigation by Kirkuk security.
In six months, police and security forces have arrested 75 ISIS militants and affiliates in Kirkuk and in camps set up for Hawija IDPs. Kirkuk authorities believe many ISIS try to disguise themselves among displaced populations in order to infiltrate the city and carry out attacks.
“They’ll try to hide with the population. Their cells will get smaller – instead of companies and platoons, they’ll go to squads and cells, much smaller elements hiding in the population," Lieutenant-General Steve Townsend, commander of the U.S.-led coalition, told reporters.
“Our Iraqi security force partners will have to engage in counter-insurgency style operations at some point and we’re already making efforts now to start shaping their training towards that next ISIS tactic,” he said.
Despite the major loss in Mosul, ISIS has proved itself capable of carrying out deadly attacks.
In early July, around 30 ISIS militants armed with machine guns and mortars crossed the Tigris River in wooden boats. They attacked the residents of Imam Gharbi some 70 kilometers south of Mosul and then retreated.
"The notion of a caliphate is gone. The dream is gone. They will revert back to their old tactics of hit and run attacks," former Iraqi foreign minister Hoshyar Zebari told Reuters. "The hardcore will keep fighting."
