Story highlights ISIS' communication, resupply lines in Iraq disrupted by airstrikes and Iraqi military action

ISIS controls vast areas in Iraq and Syria but has lost "element of surprise," analyst says

Despite gains against ISIS, group remains flexible fighting force capable of guerrilla tactics

Is the Islamic State in Iraq and Syria finally beginning to feel the pressure? The first signs are emerging that a combination of coalition airstrikes and more assertive Iraqi and Kurdish forces are forcing ISIS to change its behavior and inflicting serious losses of both territory and fighters.

Analysts tell CNN it is too early to say ISIS has "peaked." It controls vast areas of northern and western Iraq, as well as much of northeastern Syria, and exercises draconian authority in areas as far apart as Anbar in western Iraq and Aleppo province in northern Syria. ISIS also continues to pick up endorsements and pledges of allegiance from other jihadist groups, most notably in Libya and Egypt.

But there are signs that ISIS is under stress, especially in Iraq, with its lines of communication and resupply disrupted in some areas, key figures targeted in airstrikes and sources of revenue under threat.

Lauren Squires, a researcher with the Institute for the Study of War who has followed the group closely, said, "ISIS is facing the unhappy experience of losing the element of surprise."

"Mosul fell in June because no one thought Mosul would fall. ... ISIS has capitalized on the Iraqi and Western governments not expecting it to be as quick and aggressive as it has been," Squires told CNN.

Now, the group's strengths and weaknesses are perhaps better understood.

U.S. President Barack Obama said in an interview over the weekend that the next phase of the campaign against ISIS was imminent. "The airstrikes have been very effective in degrading ISIL's capabilities and slowing the advance that they were making," he told CBS's "Face the Nation."

Map: ISIS in Iraq

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"Now, what we need is ground troops, Iraqi ground troops that can start pushing them back," he said, supported by an expansion in the U.S. training and advising mission.

'Afraid to congregate'

Airstrikes by U.S. and coalition aircraft in Iraq (and U.S. strikes in Syria) are often no more than a handful per day, hampered by poor weather and the need to avoid civilian casualties. But the strikes appear to have limited ISIS' freedom of movement and destroyed some of its military hardware, as well as oil refineries that are a source of income.

Last week, Gen. Lloyd Austin, who heads U.S. Central Command, said ISIS "are afraid to congregate in any sizable formation."

"They know if we can see them, we're going to engage them, and we're going to hit what we're aiming at," he told a forum at the Atlantic Council in Washington.

Squires echoes that: "ISIS is exhibiting signs of stress, as they seldom move and maneuver in large convoys anymore. We've seen pictures of ISIS moving on motorcycles and in single trucks, as opposed to the summer, when they moved in convoys."

Recent strikes also suggest Iraqi and coalition targeting is benefiting from better intelligence about the movement of senior ISIS figures. This week, Iraqi television claimed that an aide to ISIS leader Abu Bakr al-Baghdadi was killed in an airstrike on Fallujah, days after the U.S. said it had targeted a convoy of high-profile ISIS figures near Mosul.

ISIS itself has obliquely acknowledged the impact of airstrikes, releasing a video this week about the value of trenches and underground shelters. A fighter in a shelter near Fallujah said, "If airplanes are observed in the sky, the brothers can hide themselves until the airplanes are gone," according to a translation by the SITE Intelligence Group.

But airstrikes alone can't prevent it from taking further territory. This is a group that "understands how to implement operational patience," Squires said. ISIS is also organized on a provincial basis, and local commanders have considerable autonomy, giving it flexibility and the agility to exploit enemy weaknesses quickly.

"Even if intelligence exposes the location of ISIS senior leadership so a targeted attack could kill Baghdadi, this sort of blow to the organization would actually not stall operations or affect the group's day-to-day behavior," she told CNN.

Iraqi forces regroup

The main reason for ISIS' swift advance southward through Iraq in the summer was the abject failure of the Iraqi Security Forces to put up any resistance. Bolstered by Shiite militias known as Popular Mobilization Units, the ISF is beginning to counterattack in a few places.

"Army units backed and sometimes fronted by Shia militias have undoubtedly acquired some much-needed momentum, although ISIS is showing a surprising capacity for defense under pressure," said Charles Lister, visiting fellow at the Brookings Institution in Doha, Qatar.

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A long-overdue army shakeup appears to be under way. Iraqi Prime Minister Haider al-Abadi announced Wednesday that 26 commanders were being fired and 10 more pushed into retirement in what many see as the first move toward depoliticizing the army. Abadi's predecessor, Nuri al-Maliki, had made senior appointments on the basis of political loyalty rather than professionalism and bypassed institutional constraints on his power.

A statement on Abadi's website stressed "the need to restore confidence in the security forces through real action and by combating corruption at the individual and institutional levels."

It will take time for the results of such an overhaul to show, but the Iraqi army enjoyed a taste of success this week in retaking Baiji, an important oil city of about 200,000 people north of Baghdad, which was seized by ISIS in June. On Wednesday, Iraqi television showed senior officers at the city's al Fatih mosque -- with the whole city apparently freed of ISIS fighters after a weeks-long campaign supported by coalition airstrikes and a drone attack carried out by the Royal Air Force over the weekend.

The recapture of Baiji leaves ISIS' main prize, the city of Mosul, vulnerable. While ISIS' physical control of Iraq's second largest city is yet to be challenged, it appears to have serious problems in governing. Water and electricity supplies are sporadic, according to residents contacted recently by CNN, and food prices have risen dramatically. ISIS is hunting down anyone perceived as a possible source of dissent. One Iraqi journalist contacted by CNN said six colleagues were abducted at the end of October; they have not been seen since.

Not so long ago, ISIS was advertising the social services it was providing in Mosul, posting photos showing the "prosperity" enjoyed by residents under the "shade of the Caliphate." Now, the mood is very different, with coalition airstrikes around the city pinpointing ISIS convoys and buildings.

Residents told CNN that ISIS had lost many fighters since airstrikes began and was putting pressure on families to provide fresh recruits. They added that ISIS had lost the trust of Mosul residents, and there had been several attacks on militants by newly formed citizens' "brigades."

Such anecdotal evidence is not enough to suggest ISIS is on the verge of losing Mosul, but the loss of Baiji "threatens a central corridor of the ISIS caliphate," says the Institute for the Study of War, and could isolate ISIS' military areas of operation.

Farther north, Kurdish peshmerga forces evicted ISIS from Zumar -- an important town between Mosul and the Syrian border -- at the end of last month. Kurdish commanders said more than 80 ISIS fighters were killed in the operation, but it took weeks and multiple airstrikes to expel ISIS from the town.

Western Iraq

To the west of Baghdad, ISIS still controls much of Anbar province. A month ago, the group appeared to be closing in on the capital. It had a presence in parts of Abu Ghraib not far from the international airport. There were mortar attacks on the International Zone in Baghdad, where government offices and embassies are concentrated. ISIS has also held Fallujah and much of Ramadi -- the two largest towns west of Baghdad -- for most of this year.

However, the landscape has changed somewhat following the recapture by the army and Shiite militia of the town of Jurf al-Sakhar in neighboring Babil province. Hadi al Amiri, leader of the Badr Brigade, a Shiite militia that was heavily involved in taking Jurf al-Sakhar, told Foreign Policy magazine that "With this operation complete, there are no more dangers (posed by the Islamic State) to Baghdad."

The next town in the crosshairs is Amriyat-al-Fallujah on the Euphrates River, where pro-government tribal militia forces are dug in but under frequent mortar attack from ISIS, according to Iraqi sources. If ISIS is repelled from the area, Iraqi forces potentially have a platform from which to push north into Anbar. But if ISIS takes Amriyat, the pressure on the capital will be intensified.

Iraqi efforts to reverse recent ISIS gains in Anbar province should benefit from the arrival there of U.S. trainers and advisers. According to Squires, "We assess U.S. forces are at Al-Asad airbase to train Sunni tribesmen for a counteroffensive against Hit," a nearby town seized by ISIS last month with great bloodshed after a local tribe resisted the group.

Squires sees the training mission as a "very tactical solution to a very strategic problem."

Possibly of greater significance is the declaration by the Iraqi government that it will at last begin arming Sunni tribes in the area. Meeting tribal leaders at Al-Asad on Wednesday, the speaker of the Iraqi Parliament, Salim al-Jibouri, declared, "The arming process of tribes has started today."

Such promises have been made before but, if effectively implemented, could impact the battlefield in Anbar.

The Iraqi government's dependence on Shiite tribal militias -- some of which have close links to Iran -- may yet turn out to be a double-edged sword. Lister says the "Shia militias, both during the U.S. offensive and during today's counter-ISIS operations, have rarely proven themselves effective offensive units. And when they have, they've too often proven liable to involvement in acts of brutality."

Human Rights Watch and Amnesty International have both documented atrocities by some of these Popular Mobilization Units against Sunnis.

Additionally, says Lister, the Iraqi armed forces have often shown little regard for civilian casualties "through continued artillery bombardment and the use of their new Russian (rocket launcher) systems." So winning back territory may be unsustainable if people perceive the Iraqi military as more of an enemy than ISIS.

A determined and adaptive foe

While there may be signs of progress in the campaign against ISIS, analysts point out that it can move easily from waging conventional warfare to guerrilla-style insurgency and terror attacks. This is already evident in Baiji, where a suicide bomber attacked Iraqi forces just after ISIS left the city, and in Baghdad, where almost daily suicide bomb attacks against Shiite mosques and markets keep the capital on edge.

The group is prepared to take -- and even expects -- high casualties in the pursuit of its objectives. Lister describes this as its "millenarian thinking. ... For ISIS, the idea of throwing dozens and dozens of fighters to their deaths in Kobani isn't perceived as such a high-loss move as Western strategists might think."

ISIS now appears to be committing reinforcements to the assault on Kobani despite their vulnerability to airstrikes, apparently seeing victory there as a symbol of its prowess.

"It wants to convey to the global jihadist community that it has momentum," Squires said.

No one expects ISIS to crumble in the face of more assertive Iraqi and Kurdish forces and limited coalition airstrikes. But its days of easy conquest appear to be over, and holding onto its gains present a host of challenges -- even if the senior leadership is not hollowed out by airstrikes.

In Lister's view, "ISIS faces serious long-term challenges, including maintaining its tacit alliances in Iraq and sustaining an ability to provide enough service to civilians as to ensure a necessary level of acceptance."

William McCants, fellow at the Center for Middle East Policy at Brookings, says ISIS -- in declaring a caliphate and the creation of the Islamic State -- has much to lose when it has to retreat.

"Take away the state's territory and expose its brutality and rapaciousness, and you discredit the standard bearer of the idea," he writes in Foreign Affairs. "You may even discredit the idea itself."