After a brief pause in the effort to liberate the western half of the city from Islamic State control, the American-led operation to retake Mosul has resumed in force.

The pause came after a Mar. 17 coalition air raid that killed up to 270 civilians in Mosul’s al-Jadid district. While still under investigation, US Central Command confirmed a strike was carried out “at the location corresponding to allegations of civilian casualties.”

Spokesman for the US-led coalition, Col. John Dorrian, wouldn’t say when the investigation would be concluded, but said the coalition was carrying out “the most precise air campaign in history.”

The Pentagon acknowledges a total of 352 civilian casualties caused by US operations in both Iraq and Syria—only 14 of which they claim were caused by the Mar. 17 raid—but activists and international monitoring organizations say the figure is much higher. Airwars, a monitor based in London, estimates that the coalition has killed at least 3,000 civilians overall since US operations against ISIS began in Syria in 2014.

The Mar. 17 strike, however, was one among thousands in the ongoing endeavor to expel ISIS from Mosul.

An intense American bombing campaign supports the predominantly Shia Iraqi Security Forces, Kurdish and other militiamen, as well as US troops on the ground, with up to 500 bombs dropped on the city per week in March.

In addition to around 100,000 Iraqis, an estimated 6,000 American military personnel are now stationed in Mosul, as well as over 3,500 Pentagon contractors. Exact troop numbers are difficult to determine, however, since the administration stopped disclosing troop deployments in Iraq and Syria in March. Even when official tallies were available they were not complete, as certain categories of personnel were excluded from the figure.

The expanding presence of Shiite militias in Mosul and the surrounding rural areas has some Iraqi officials concerned about post-war reconciliation with members of the country’s Sunni minority. While parts of the Hashd al-Shaabi, or the Popular Mobilization Units (PMF), were recognized as an official paramilitary force by the Iraqi government in December 2016, the growing network of militia checkpoints has become a source of anxiety for some Sunni Arab villagers.

Retributive sectarian murders were commonplace in the civil war that broke out in the years after the 2003 American invasion, and some excessive militia violence was reported earlier in the fight for Mosul, but the extent of the abuses are difficult to verify in a politically complex environment.

“When the western ISIL positions were first taken by the [Iraqi government], it was a merciless and bloody struggle and the Shia PMFs did a lot of indiscriminate killing and executions,” said a Sergeant 1st Class in the US Special Forces with extensive experience in Mosul who wished to remain anonymous.

However the Sgt., who finished his seventh tour of duty in the Middle East last October, noted that many of those reports were “heavily exaggerated by ISIL and ISIL’s supporters in Baghdad, as well as by Sunni military leaders who tried to co-opt these incidents to make a push for leading the Mosul liberation.” ISIL is an alternative acronym for the Islamic State.

If unwanted Shia military presence in historically Sunni areas continues long after ISIS is expelled from Mosul, Iraq may have a prolonged conflict on its hands.

Timetables for the recapture of Mosul have proven inaccurate since operations commenced last year. Sirwan Barzani, a brigadier general in the Kurdish Peshmerga, initially told CNN the battle would last two months, but after six months of heavy fighting officials appear to have given up on time estimates.

While recent reports put 30 percent of West Mosul back under the control of the Iraqi government—and Iraqi officials estimate less than 1,000 ISIS militants left in the city—the battle against the Islamic State in Iraq is likely still far from over.

“Tal Afar is a huge insurgent stronghold, just like Western Mosul,” the Special Forces Sgt. said. “The Kurds and the [Iraqi government] have been hammering that in advance because they predict that the insurgents will stage another Alamo there. They are of course correct, and quite a lot of damage has been caused in Tal Afar as a result.”

Despite heavy losses among the Iraqi Security Forces, civilians have perhaps seen the worst of the fighting in Mosul. Civilian casualty numbers for the city are hard to come by, but according to data compiled by Iraq Body Count, more than 8,600 civilians are estimated to have been killed in Nineveh Province, where Mosul is located, since the coalition began its current operation in October 2016.

Most of the deaths in Mosul have been caused by ground combat, but American bombs have also contributed to the figure. The excessive civilian death toll has recently prompted the US-led coalition to reevaluate its use of airpower in Mosul’s densely-populated western districts.

American casualties have been low by comparison, with two fatalities since US operations began in Mosul last year and “some number” wounded, Pentagon officials say.

Overall, 355,000 residents have been displaced from the city since 2016 due to the conflict, according to Iraq’s Ministry of Displacement and Migration, while the UN estimates that another 400,000 still live in ISIS-held West Mosul. Severe food shortages and even deaths from starvation were also reported from Mosul’s Old City last March.

The Islamic State first captured Mosul in June of 2014, where, in the city’s Great Mosque of al-Nuri, ISIS leader Abu Bakr al-Baghdadi declared the creation of its caliphate. While it had major successes taking territory in its initial surge in 2014, the militant group has recently seen setbacks in both Iraq and Syria, its primary theaters of combat.

Sporadic fighting has taken place in Mosul since it fell to ISIS, but the American-led coalition began providing air support in a 2015 offensive intended to cut off ISIS supply lines and capture the areas surrounding the city. Iraqi forces were expected to launch a full-scale operation to retake Mosul that spring, but the Islamic State takeover of Ramadi, another key Iraqi city, threw that operation off schedule.

Since then, American involvement has steadily ramped up, with increased air support and US military advisors and ground troops deployed to the city in 2016 at the start of the current operation. As the it pushes into Mosul’s western districts, the coalition inches closer to the Great Mosque, perhaps symbolic of the slow but steady erosion of the Islamic State’s foothold in the city.

What’s next for Mosul? The answer to that question will largely depend on future American policy, but with Iraq’s immense political and demographic complexities, conflict appears set to continue, even after the Islamic State is pushed out of its urban strongholds and turned back into an insurgency.