Iraqi army soldiers stand with their weapons in center of Fallujah. Thaier Al-Sudani/Reuters The Iraqi Security Forces (ISF) entered the ISIS stronghold of Fallujah on Friday, recapturing the city's main government building and hospital with minimal resistance from the jihadists.

Tim Arango, The New York Times' Baghdad bureau chief, described the government forces' gains as "rapid and unexpected," noting that ISIS had abandoned checkpoints and was not preventing civilians from leaving the city. The group had, until now, been using civilians as human shields.

"The rapid, and unexpected, gains suggested a shift in tactics by the Sunni extremists of the Islamic State, also known as ISIS or ISIL, or perhaps a sign of their weakness, as they abandoned their dug-in positions and regrouped in western neighborhoods," Arango reported.

Iraqi commanders inside Fallujah were optimistic about the offensive's success in driving the militants out of the city, which fell to ISIS in 2013, for good. And most analysts agree that the recent gains are significant.

But many are at best cautiously optimistic, due to what ISIS' quick retreat might indicate about the group's shifting priorities and tactics and the role Shiite militias have played in liberating the overwhelmingly Sunni city.

Patrick Martin, an Iraq expert at the Institute for the Study of War, said that as ISIS suffers battlefield losses, they will begin to shift their emphasis away — at least in the short term — from territorial gains. And they will move toward more terror attacks and insurgent-style tactics.

'They will first and foremost begin to emphasize resurging in recaptured areas, as they did in Karbala in early June," Martin told Business Insider on Friday. The group took responsibility for a car bombing on the first day of Ramadan that killed 10 people in Karbala, in Hayy al-Muwatafin, just under two miles from a Shiite shrine.

It was the first successful ISIS attack in Karbala since October 2014, according to the Institute for the Study of War. Iraqi officials have expressed concern that ISIS militants might try to escape from Fallujah, hiding in the wave of civilians who have fled the fighting.

"Within Iraq, we will see them shift more toward attacking civilian tactics" as they lose territory, Martin said. "And outside the country, there will likely be a renewed emphasis on sending foreign recruits to Libya," where the group has been trying to establish a foothold.

Sunni 'distrust' of Baghdad

A wave of suicide bombings targeting marketplaces and recreational areas in Baghdad's Shiite districts killed and maimed hundreds of civilians throughout May, as Iraqi forces were approaching the ISIS-held city of Mosul further north.

The attacks prompted the government, against Washington's advice, to refocus its anti-ISIS campaign to Fallujah, from where Prime Minister Haider al-Abadi believed many of the bombers were coming.

But Abadi's decision to allow the state-sponsored, Shiite-dominated Popular Mobilization Forces (Hashid Shaabi) to play a role in liberating the overwhelmingly Sunni city of Fallujah has contributed to experts' wariness about the campaign's long-term success.

"Sunni distrust of Baghdad and the US is reinforced by allowing Shia militias to play any role in our ISIS strategy in Iraq," Michael Pregent, an adjunct fellow at the Hudson Institute, told Business Insider on Friday.

Pregent was an embedded adviser with a Peshmerga battalion operating in Mosul between 2005 and 2006 and a former intelligence adviser to Gen. David Petraeus from 2006 to 2011.

"What matters most is how Sunnis are treated by the security forces and the militias they work with," Pregent added. "And they are being rounded up and tortured by militias with uniformed Iraqi army and police watching."

Shi'ite fighters stand with their weapons on the outskirts of Saqlawiya. Thomson Reuters

Human Rights Watch and other watchdog organizations have published reports accusing the Shiite militias of committing human-rights abuses against Sunni civilians living in and around Fallujah. The militias have accused the citizens of being Islamic State sympathizers.

“Some took videos of us being tortured and laughed,” Mohammed Aziz, 26, told The Wall Street Journal last week. “They all screamed at us: ‘You supported Islamic State. You deserve this. This is for Speicher. This is for our brothers.’”

Hundreds of young Sunni men have disappeared from Fallujah since the offensive began, The Journal reported. Their families suspect they have been kidnapped by the Shiite militias.

Fighters from the Iraqi Shi'ite Badr Organization walk past a poster depicting images of former Iraqi President Saddam Hussein on the outskirts of Fallujah. Thaier Al-Sudani/Reuters "We are simply resetting the conditions that led to ISIS to begin with — a disenfranchised Sunni population now punished and hardened in their opposition to Baghdad," Pregent said.

Martin, of the ISW, largely echoed Pregent's analysis.

"It is necessary for us to keep Iranian proxy actors from participating, particularly if Federal Police forces are involved," Martin said.

The Iraqi government has denied that any elements of the PMU are backed by Iran. But elements of the Badr Organization, an Iranian proxy militia, have been spotted inside Fallujah's southern neighborhoods, alongside Federal Police units. Iran's powerful military commander, Qassem Soleimani, was also photographed outside Baghdad in May.

'The focus is on the symptom'

Since June 2014, when ISIS overran Iraq's second-largest city, Mosul, much of US President Barack Obama's dealings with Baghdad have revolved around formulating a cohesive strategy to halt the jihadists' momentum in Iraq and Syria.

But experts say the Obama administration's narrow focus on consolidating a partnership with Abadi — whom many within Iraq see as beholden to Iran — has resulted in a one-dimensional policy. It focuses too heavily on one manifestation of Iraq's political instability, ISIS, rather than its root cause, which is widespread incompetence and corruption.

"The message to the Iraqis has been to focus on the short-term problem that this president would like solved by January," Doug Ollivant, a former military planner in Baghdad and senior fellow at the New America Foundation, told The Washington Post in May. "The focus is on the symptom and not the root cause of the problem."

Followers of Iraq's Shi'ite cleric Moqtada al-Sadr storm Baghdad's Green Zone after lawmakers failed to convene for a vote on overhauling the government, in Iraq, April 30, 2016. REUTERS/Khalid al Mousily Protesters stormed Iraq's heavily fortified Green Zone in early May, demanding reform that would replace a political class selected according to their sect and ethnicity — a "quota system" implemented by the US-led coalition after the invasion — with technocrats chosen solely for their professional qualifications.

Mounting discontent with the central government, experts say, is another reason why a political foundation establishing who will govern ISIS territory once the jihadists are driven out is essential to any plan that aims to decisively destroy them.

"The bigger issue has always been what happens after ISIS leaves the stage," Stephen Biddle, who has worked in groups under Generals Stanley McChrystal and David Petraeus forming US counterinsurgency policy, told Business Insider on Friday.

"It's long been clear that they'd eventually fall," Biddle, who is now an adjunct senior fellow for defense policy at the Council on Foreign Relations and professor of political science and international affairs at George Washington University, added. "But there's no apparent viable governing model to replace them. So the same underlying sectarian tension that fueled their rise is likely to destabilize any successor government."

A fighter from the Iraqi Shi'ite Badr Organization holds his rifle in an underground tunnel built by Islamic State fighters on the outskirts of Fallujah, Iraq. Thaier Al-Sudani/Reuters Ali Khedery, the longest-serving US official in Iraq, largely agreed. He argued that Washington's tendency to emphasize short-term victories over extremists without regard for political and sectarian realities comes at the expense of long-term stability.

"It is important to point out that this is at least the fourth time Fallujah has been 'liberated' since 2003 — 2003; 2004/2005; 2007/2008; and today," Khedery, who served as a special assistant to five US ambassadors and as a senior adviser to three heads of US Central Command, said.

"It subsequently fell to insurgents again and again precisely because the only meaningful and sustainable drivers of peace — national reconciliation and power-sharing among Iraq's major ethnosectarian communities were never addressed and never achieved."

Washington's "lack of understanding of this reality — and thus its continued missteps — is stoking this holy war by facilitating US support to both sects across various fronts," Khedery added. "This has only made the world a much more dangerous place."