BEIRUT, Lebanon — In the early months of summer last year, a string of military victories by fighters belonging to what was then known as the Islamic State of Iraq and Syria shocked the world.

Over the preceding year they had steadily gained territory in the chaotic battlefields of the Syrian civil war, and suddenly they looked set to do the same in Iraq.

The cities of Haditha, Samarra, Mosul, and Tikrit fell in quick succession as Iraqi army forces fled south toward Baghdad. Pausing briefly to declare its caliphate in late June, the newly renamed Islamic State (IS) pushed on in earnest.

Its fighters captured Sinjar in early August, massacring civilians belonging to the minority Yazidi sect and besieging thousands more who fled to nearby Mount Sinjar.

Then they pushed farther east still, coming within 45 kilometers of the Kurdish capital, Erbil.

With the massive civilian population of Erbil under threat, and Yazidis dying of starvation on the mountain, Barack Obama authorized "targeted" strikes against IS on August 7, 2014.

Over the next month, U.S. airstrikes become more frequent, and what Obama initially described as a "limited" operation transformed into a sustained effort to "degrade and ultimately destroy" IS involving a broad coalition of Western and Arab countries.

One year since that intervention, the U.S. and its allies have spent billions of dollars and carried out 5,885 airstrikes, killing an estimated 15,000 IS fighters and some 450 civilians, according to one monitoring group.

But just how successful has the campaign against IS really been?

Lt. Gen. William C. Mayville Jr., Joint Staff Director of Operations Director of Operations, speaks about airstrikes in Syria during a briefing at the Pentagon September 23, 2014 in Washington, DC. | (Brendan Smialowski/AFP/Getty Images)

Squeezing a balloon

U.S. military officials claim slow and steady progress against IS, pointing to the recapturing of territory in northern and central Iraq.

Speaking back in March, Pentagon spokesman Col. Steve Warren said that IS "no longer has complete freedom of movement in roughly 25 percent of populated areas of Iraqi territory where they once operated freely."

"Their caliphate is in the process of shrinking," he told reporters, pointing to the group's loss of dominance in Babil, Diyala, Nineveh, Salahadin, and Kirkuk.

The UK-based research center IHS says that IS has lost almost 10 percent of its territory in the first six months of 2015, leaving its total area of control at some 82,940 square kilometers (32,000 square miles).

The trouble with the U.S. assessment, though, is that the group's capability hasn't been significantly degraded as a result.

"It's like squeezing a balloon. You can squeeze in one part but it only increases the pressure in other areas," says Shiraz Maher, a senior fellow at the International Centre for the Study of Radicalisation (ICSR) at King's College London.

"There have been some notable fighters killed, but the airstrikes have not really dented the leadership of IS or affected its operation capacity. They are still a very resilient movement on the ground."

The Islamic State has suffered setbacks in some areas, but it has also demonstrated that it has the capacity to make inroads in others. The ancient city of Palmyra fell to the militants in May after Syrian government forces were routed, and there are signs they plan to move south.

IS has consolidated control of its key cities, Mosul and Raqqa, and looks set to remain there for some time.

There are also strong signs that the group's overall numbers have not diminished significantly, despite many fighters being killed. Experts estimate that IS gains around 850 to 1,250 new recruits per month travelling to Syria and Iraq from abroad, although there are signs that may be changing due to a greater effort by Turkey to police its border.

A picture taken on November 6, 2014 from the Turkish city of Mursitpinar shows smoke rising during a shelling by Islamic State militants to the Syrian city of Kobane. | (Aris Messinis/AFP/Getty Images)

Friends and enemies

One of the problems with the U.S. strategy is that airstrikes can only do so much. Without its own combat soldiers on the ground, the U.S. has relied on a number of different forces to take back territory. While these forces share a common enemy in IS, many of them are also bitter enemies.

The United States has different levels of commitment to each of these groups, largely dependent on its own aims. Where cooperation between forces on the ground and the coalition is high, airstrikes have been effective.

The battle for the strategic Syrian border town of Kobani — controlled by the Kurdish YPG — was perhaps the best example of this. In late September, when Kobani was close to being overrun, the U.S. intensified its bombing campaign. Kobani and its surroundings were the target of around half a dozen airstrikes every day, according to The Associated Press, and IS was eventually forced out of the city.

The airstrikes had a "crucial" impact on the defense of Kobani, according to Alan Semo, a member of the foreign affairs committee of the Democratic Union Party (PYD) — the largest political force in Kurdish Syria and the sister organization of the YPG.

Airstrikes also played a key role in the YPG's recapture of Tal Abyad, another town on the Turkey-Syria border, in June this year. More recently, YPG forces recaptured the city of Hassakeh, a mixed Arab-Kurdish city in Syria's north, with the help of coalition airstrikes.

A similar level of cooperation exists between the coalition and the Kurdish peshmerga in Iraq. Its commanders are in close contact with US advisors in the country and have the ability to call in coalition airstrikes. This has proven useful in taking back areas of Kirkuk province.

U.S. cooperation with Iraqi security forces is more complicated. Due to the Iraqi government's reliance on Iran-backed Shia militias — many of whom cut their teeth fighting American troops during the U.S. occupation of Iraq — U.S. airstrikes have often come with conditions.

Iraqi forces, whom the U.S. has assisted with training and equipment, are currently advancing slowly in Anbar province with the help of U.S. airstrikes. But it remains to be seen whether they can make serious headway.

With the exception of the Kurds, and the initial mission to prevent the fall of Erbil and assault on Mount Sinjar, it would be a stretch for any of the groups fighting IS to claim that they have dealt it a significant blow. It may even be that it has grown stronger, according to Hassan Hassan, non-resident fellow at the Tahrir Institute for Middle East Policy and author of ISIS: Inside the Army of Terror.

"Any tactical territorial loss the group made since the air campaign began has compensated for it in strategic currency on the ground," says Hassan. "These losses are meaningless as long as the group is establishing a foothold not only in the territories under its control but in other areas where it is quietly expanding."

A map showing the extent of ISIS zones of control, attack, and support throughout Syria and Iraq. | (Institute for the Study of War/Courtesy GlobalPost)

The long game

A significant weakness in U.S. strategy has been the failure to build any significant Sunni force to counter IS, which adheres to an extreme interpretation of the same branch of Islam.

IS was able to sweep so easily through western Iraq in large part because of the deep resentment the area's Sunni community felt toward Iraq's Shia-dominated central government.

While the majority of Sunnis do not support the terrorist group's ideology, many found them preferable to the largely Shia security forces who held the area previously, whom they complained had harassed and abused them for years.

Since it took control of the Sunni areas of western Iraq, IS has sought to portray itself as the sole protector of Sunnis. It is for this reason that most experts agree a viable Sunni force — rather than the Shia dominated security forces — is essential for retaking and maintaining control of these areas. A similar strategy showed results for the US during its occupation of Iraq; the 'Sahwah' (Arabic for Awakening) movement, made up of Sunni tribesman whose wages were paid by the U.S., helped drastically reduce violence in the country after 2006 and made it increasingly difficult for Al Qaeda in Iraq, the Islamic State's predecessor, to operate effectively in Anbar province.

Despite this, U.S. support for Sunni tribes in Iraq has been slow. A lack of distrust on both sides is behind the delay. US military leaders are wary that any weapons they send might end up in the hands of IS fighters, while Sunni tribes fear the US is too close to the Shia government in Baghdad.

The lack of support for Iraqi Sunnis opposed to IS, combined with reports of Shia militias carrying out abuses in recaptured Sunni towns, will do little to win their confidence.

This lack of strategic foresight also extends into Syria.

Most moderate rebel groups in Syria have been forced out by better-funded and more extremist factions as the war has dragged on. A $500 million US program to train and equip Sunni Syrian rebels to fight IS has vetted a grand total of 60 fighters. Many of them have already been captured or killed.

The country's majority Sunni population rightly sees Bashar al-Assad as a greater threat than IS. Syrian government barrel bombs — usually oil drums filled with explosives and metal — have been responsible for tremendous civilian casualties.

But despite repeatedly stating its commitment to the removal of Assad, the U.S. has refused to use its firepower against the Syrian army.

Syrians deeply resent that the US is bombing IS while leaving alone a regime that has been responsible for far more deaths, says ICSR's Maher, whose work focuses on tracking foreign fighters in the conflict.

"The idea that a coalition drone is sharing the same airspace as a government helicopter dropping barrel bombs on a civilian area, people rightly see as favoring Assad," he says. "They see them hitting ISIS but not doing anything about barrel bombs."

"Massive levels of resentment built up, even among remaining elements of Syrian civil society who remained somewhat predisposed to the West. There is a sense now that [coalition countries] are in it for themselves."

This photo taken on June 9, 2015, shows Iraqi Shia fighters ride in the back of a vehicle in the town of Baiji, north of Tikrit, where they are fighting alongside Iraqi government forces against the Islamic State. | (Ahmad al-Rubaye/Agence France-Presse)

Unintended consequences

These feelings are compounded by the unintended consequences of a sustained aerial bombing campaign.

More than 450 civilians have been killed by coalition airstrikes targeting IS, according to a recent study by Airwars, an independent investigative project. The U.S. government has so far only accepted responsibility for two civilian deaths caused by the nearly 17,000 bombs and missiles dropped on Iraq and Syria.

Each and every civilian death serves as a powerful propaganda tool for IS, according to Maher.

"It allows them to say: ‘Look, here is a dead child, killed by the West. Isn't this the same as Assad? If its right to fight Assad because he kills children then it's right to fight the West for the same."

The U.S. and its allies can point to certain figures and paint a rosy picture of the anti-IS campaign. But numbers can be misleading. The Islamic State has allegedly lost more than 15,000 fighters and a large amount of territory without its overall integrity being threatened. It seems likely that the fight will continue for some years to come.

"The anti-ISIS coalition has failed to understand basic facts about the group and the terrain in which it is fighting," says Hassan.

"The reality on the ground indicates that ISIS is here to stay, for the foreseeable future."

This article, by Richard Hall, originally appeared at GlobalPost.