Nytimes.com | By BEN HUBBARD

BAGHDAD — The extremists of the Islamic State appeared unstoppable after their sudden blitz through Iraq this summer, with its battle-hardened fighters continually raising their black flag over newly conquered areas.

Today, roughly a third of Iraq is dotted by active battle fronts, with intense fighting and occasional Islamic State victories. But analysts also say the days of easy and rapid gains for the jihadists may be coming to a close in Iraq, as the group’s momentum appears to be stalling.

The international airstrike campaign against the Islamic State, also known as ISIS or ISIL, has clearly played a role in slowing the Sunni Muslim group’s advance. But analysts say other factors are having a major effect, including unfavorable sectarian and political demographics, pushback from overrun communities, damage to the group’s financial base in Syria and slight improvements by ground forces in Iraq.

Across the territories the Islamic State holds, the group has overhauled its operations. Bases and hospitals have been evacuated and moved to civilian homes that are harder to identify and bomb, Iraqi officials said. Fighters who used to cross the desert in convoys now move in small groups or by motorcycle.

“The airstrikes from the coalition have been very helpful, and now the ISIS fighters are confused and don’t know where to go,” said Maj. Gen. Hamad Namis al-Jibouri, the police chief of Salahuddin Province in Iraq, where a combination of government security forces and Shiite militias have been fighting the jihadists near the town of Baiji. “They have also raised the spirits of the groups on the ground that are fighting ISIS.”

Still, airstrikes alone cannot achieve President Obama’s goal to “degrade and ultimately destroy” the Islamic State, analysts say. And they have not been the only reason the group’s advance has seemed to slow.

One main factor in the shift has been demographics. ISIS thrives in poor, Sunni Arab areas that have lost their connection to the central state. The Sunni-led uprising against President Bashar al-Assad in Syria opened up such areas there. And the neglect of such areas in Iraq during the tenure of former Prime Minister Nuri Kamal al-Maliki made them an opportunity for the jihadists.

But after months of steady expansion, the Islamic State has taken most of these areas in Iraq while failing to seize areas with non-Sunni populations. And although it could still expand in Syria, the group also faces resistance from rival rebel groups there.

“ISIS can only expand in areas where it can enter into partnerships with the local population, and that largely limits the scope of the expansion of ISIS to Sunni, disenfranchised areas,” said Lina Khatib, the director of the Carnegie Middle East Center in Beirut.

It is in Iraq, where coalition forces began bombing in August, that the Islamic State has lost the most ground. In recent weeks, combinations of Iraqi government units, Kurdish pesh merga forces, Shiite militias and armed Sunni tribesmen have seized the Rabia crossing with Syria; taken back the area of Zumar in the north and Jurf al-Sakr south of Baghdad; opened crucial roads in the country’s center; and held off Islamic State advances elsewhere.

For the first time since the jihadists seized Mosul and much of northwestern Iraq in June, an Iraqi military vehicle can drive from Baghdad to the northern city of Erbil on a main highway.

Hisham Alhashimi, an Iraqi researcher and an expert on the Islamic State, said those changes had broken up the group’s territory, making it harder for it to move its forces and for its couriers to relay messages among the leadership and the field commanders.

And indications have emerged that Sunni populations in some areas it controls are trying to undermine it. In Diyala Province, northeast of Baghdad, ground forces have cut the group’s supply lines and killed a number of its local leaders with the help of tips from angry residents, security officials there said, speaking on condition of anonymity under government protocols.

Others say the group’s own rhetoric has left it vulnerable.

What differentiates the Islamic State from Al Qaeda and other jihadist groups is that it claims to have re-established the Islamic Caliphate, making its commander the spiritual leader of Muslims everywhere. Very few Muslims abroad agree, and the group’s argument would further fall apart if its fighters went underground.

“So central to this group’s appeal is its ability to keep expanding,” said Noah Bonsey, a Syria analyst with the International Crisis Group. “But as soon as that stops, the whole narrative is less convincing.”

While the group appears to have lost no ground in Syria, the air campaign has forced it to leave its headquarters in former government buildings and lighten its patrols in the city of Raqqa. And strikes on oil wells and small refineries run by the Islamic State have undermined its economic base, making fuel prices rise.

Over the last week, Islamic State fighters have been struggling with government forces for control of natural gas fields in Homs Province, facilities that are unlikely to be bombed because they fuel electricity plants.

While airstrikes have weakened the Islamic State, its adaptations will make it even harder to fight without effective ground troops, Mr. Alhashimi said.

Its fighters now move in small groups, making them less vulnerable to air power. And instead of storming into towns with overwhelming force, the group has begun establishing sleeper cells in areas it wants to seize.

“It used to be that a force would come from the outside and attack a city,” Mr. Alhashimi said. “Now the forces rise up from inside the city and make it fall.”

It has certainly not been all setbacks for ISIS. While the various Iraqi ground forces have generally grown more effective, they are still lacking in many parts of the country, including Anbar Province, a vast and predominantly Sunni Arab region that abuts the capital.

Last month, Islamic State seized the Anbar town of Hit and has since been killing members of the Albu Nimr tribe, which resisted its advance. The Iraqi human rights ministry said this week that more than 300 tribe members had been killed.

Because of Iraqi’s sectarian dynamics, most agree that the government cannot send Shiite forces to fight in Anbar. The result has been a delayed, anemic attempt to push back ISIS there.

“The executions continue, and the support is weak,” said Naim al-Gaood, an Albu Nimr leader who has spent recent weeks asking Iraqi officials for arms support while receiving nearly daily reports of new killings from home. “All we are asking for is supplies to protect people from getting killed and food to keep them from starving.”

The Islamic State faces even less resistance in Syria, where government forces and the rebels are exhausted from three and a half years of civil war. A covert program by the United States to arm select rebel groups has made little difference, and a Pentagon program to train 5,000 fighters a year is still in the planning stages.

In many areas dominated by the Islamic State, residents still cannot imagine a force that can push it out.

“There are a few guys who try to launch attacks on them or shoot at them, but there is no force that can really challenge their control,” said an activist reached through Skype in the eastern Syrian city of Deir al-Zour.