The Battle to Free Mosul From ISIS Has Begun. It Won’t Be Easy.

Iraqi forces launched a highly-anticipated operation on Monday to reclaim the city of Mosul from ISIS, marking a critical turning point in the international campaign to roust the jihadist group from its strongholds in Iraq and Syria, but raising fears for the city’s population during what could be a long and bloody battle.

Mosul is both a strategic and symbolic center in the fight against ISIS. The city is the largest under the jihadists’ control, a base for military and propaganda operations as well as the living embodiment of the group’s claim that it has established an Islamic State. If Iraqi forces succeed in retaking the city, it would represent a body blow to ISIS’ larger project of capturing territory and imposing its puritanical vision of government and life on entire populations.

The battle for Mosul also represents an inflection point for Iraq itself. The operation is a critical test of the Iraqi state’s capacity to recapture, secure, rebuild, and govern its territory. The battle also offers the chance for Iraqi forces to reverse the embarrassing collapse of June 2014, when ISIS swept into the city meeting little resistance, forcing thousands of Iraqi soldiers to flee in an alarming illustration of the failure to fully rebuild a credible fighting force after the Iraqi military was dismantled by occupying American forces in 2003.

Read more: The Next War for Iraq

Prior to the ISIS takeover, Mosul was Iraq’s second or third largest city, a microcosm of Iraq’s hugely diverse population and a home for Arabs and Kurds, Sunnis, Shiites, and Christians. There are at least 600,000 people still living in the city, although some estimate the population is much higher. Arrayed against the Islamic State is a broad coalition that also reflects the political, regional, and sectarian divisions that have torn Iraq apart since the U.S.-led invasion in 2003. The current military coalition includes the Iraqi military and special forces, allied Sunni Arab fighters, Shiite-dominated militias, and Kurdish troops, all backed by the airpower of the U.S.-led international military coalition against ISIS, as well as more than 5,000 U.S. troops on the ground in Iraq.

Iraq’s Prime Minister Haider al-Abadi announced the operation in an early morning televised speech. Sitting behind a large desk and flanked by top military officials, he addressed the people of Mosul. “God willing, soon we will meet on the soil of Mosul, to celebrate the liberation and your salvation, and we will live together once again, with all our religions and sects.”

The Mosul offensive is expected to last for several weeks, if not longer. It will likely begin with a maneuver to partially or completely encircle the city, followed by an invasion of the city itself. If ISIS’ forces choose to dig in, the urban terrain of the city could offer favorable ground for them to inflict casualties on the invading forces. The jihadist forces include Iraqis, Syrians, and some expatriate fighters from across the Arab world, Europe, and elsewhere. They are expected to employ the signature tactics that they have used with devastating effects elsewhere in Iraq: tunnels, booby traps, and car and truck bombs.

Aid agencies are warning that the battle could trigger a humanitarian nightmare, as there is insufficient infrastructure to accommodate the huge wave of people who are expected to flee the fighting. Currently, refugee camps in the area can accommodate an additional 60,000 people, with more being built. But according to the United Nations, as many as a million people could be displaced in a worst case scenario, with some agencies predicting that as many as 200,000 could flee in the initial days of the fight.

The current offensive has been months in the planning, with preliminary operations to take villages and towns south of Mosul underway since last March, and more than 150,000 people have already fled those areas. Wolfgang Gressmann, the Iraq director for the Norwegian Refugee Council, said in a statement, “We fear the humanitarian consequences of this operation will be massive.”

Once Mosul is recaptured, Iraqis will face the generational task of rebuilding and repopulating the city and repairing the social fabric in a country devastated by more than 13 years of conflict. Other Iraqi cities recaptured from ISIS have been left in ruins, with little hope of immediately restoring normal life there. The rival forces now cooperating to take Mosul share little beyond their commitment to fighting ISIS.

The Shiite-led militias that command huge influence in Iraq stand accused of rights violations against Sunni Iraqi civilians. The role of Kurdish militias known as the Peshmerga is critical, but the central government in Baghdad faces a long struggle with the Kurdish regional government based in the city of Erbil over how the land and resources of northern Iraq will be divided and governed. Once the jihadists are routed, many Iraqi’s fear that a new civil war could follow as the rival factions vie for power.

But for now Iraq’s rival armed forces are united, faced with the immediate task of seizing the most important urban area under ISIS control. A defeat in Mosul would represent the most significant setback to date for the Islamic State, which is reeling from a long series of losses in both Iraq and Syria. Iraqi forces have reclaimed territory in Tikrit, Ramadi, and most recently Fallujah from Iraq, leaving Mosul as the only major city under jihadist control. In Syria, the group is also losing ground to Kurdish-led militias and to a new offensive by Turkish-backed Syrian rebel groups. On Sunday, the Turkish-supported groups retook Dabiq, a small town in northern Syria that carries symbolic importance because it is the place where the jihadists’ theology predicts the apocalypse.

As ISIS’ empire collapses, it continues to lash out at softer targets far beyond the battlefields. The group claimed responsibility for attacks in Baghdad that killed some 55 people on Saturday. The deadliest of those was a suicide bombing at a Shiite religious gathering that killed at least 41 people and injured another 33, according to Reuters. The killings offered a lethal illustration of Iraq’s longer-term challenge of combating militancy, resolving sectarian differences, and repairing a country exhausted from war.