Donald Trump’s grasp of international politics may sometimes be vague, but even he has noticed that there is a fight coming for Mosul. U.S. and Iraqi leaders have been signaling for weeks that a major offensive is in the offing against Iraq’s second city, by far the largest urban center held by Islamic State.

Republican presidential nominee Donald Trump at the town hall debate at Washington University in St. Louis, Missouri, U.S., October 9, 2016. REUTERS/Jim Young

In Sunday’s presidential debate with Hillary Clinton, Trump questioned why U.S. officials and their Iraqi counterparts had been so explicit in forewarning the Islamist group an assault was in the works. “Why can’t they do it quietly?” he said. “Why can’t they make it a sneak attack?”

In fact, the United States and its regional allies haven’t been above deception when it comes to indicating where they aim to attack next. Earlier this year, they signaled extensively that an assault on the IS capital of Raqqa, Syria was imminent. The group responded accordingly – only to find the true target was Manbij, which ultimately fell to U.S.-backed mainly Kurdish forces in August after three months of hard fighting.

It’s possible something similar may be going on now. Sooner or later, however, a push will be made on Mosul. Beating Islamic State there would deal a serious blow to its legitimacy, casting doubt on the idea that its so-called caliphate might enjoy divine protection. Reasserting control over the predominantly Sunni city is equally as important for Iraq’s multiethnic but largely Shi’ite-dominated government.

It would be almost impossible to prepare for such a major assault in secret. Suggesting the battle is coming, however, is also a central part of the strategy. The target audience is not only Islamic State commanders, some of whom are already reported to be fleeing. It is the estimated million or so people still within the city, whose behavior may be just as important as the combatants in shaping the final conflict.

Governments and armies have always hated having to fight in cities. The defender invariably benefits from battles amid narrow streets and frightened civilians. As we have seen in Syria’s Aleppo, such campaigns can easily turn into long and inconclusive bloodbaths.

When IS first took Mosul in June 2014, its fighters were ruthlessly well-prepared. Iraqi soldiers lacking heavy antitank weaponry had little defense against dozens of heavily armored truck bombs that annihilated checkpoint after checkpoint. The group sent propagandist videos of hostage decapitations to the smartphones of troops and militiamen, many of whom simply melted away.

For the Iraqi government, U.S.-led coalition and assorted other players such as the Iraqi Kurds, using such tactics to get the city back is obviously out of the question. So is using the kind of indiscriminate force Syrian President Bashar al-Assad and his Russian allies have deployed in Syria. Instead, the battle for Mosul will be in many ways a testing ground for what the allies must hope will be a rather more nuanced form of warfare.

For the United States, operating against IS in Iraq is in many ways easier than in Syria. Washington’s alliances are clear, and it knows it wants to win back the country for the government in Baghdad. That makes Mosul an easier target than the smaller IS capital of Raqqa, at least diplomatically speaking. Should Raqqa fall, it might well be retaken by Assad. And if U.S.-backed largely Kurdish Syrian rebels were to do the job, that might deepen existing differences with Turkey.

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It’s unclear, however, whether the Mosul offensive will work. Getting the city back without a humanitarian catastrophe – and without further exacerbating existing ethnic and sectarian divisions – will prove a colossal challenge, one Islamic State will do everything it can to make harder.

Iraqi government forces, experts say, have improved dramatically in the two years of war with IS, particularly in the difficult art of close-quarter street battles. U.S., British and other foreign special operations forces have been fighting in Iraq and similar environments for more than a decade, the Iraqi Kurds even longer.

The battle plan for Mosul has been two years in the making. In many cases, the plan is for units from different ethnic backgrounds to be assigned to the most similar neighborhoods of the city. That means those leading the assault into predominantly Sunni areas – which make up much of Mosul – should come from broadly similar backgrounds. So should those tasked with minority Kurdish, Shi’ite and other ethnic areas.

That’s a dramatic difference from some of the previous battles in the war, most notably the recapture of Saddam Hussein’s hometown of Tikrit in 2015. That was largely spearheaded by Shi’ite militia units and heavily coordinated by Iran’s Revolutionary Guard. In the days and weeks afterwards, reports of atrocities committed against the majority Sunni population helped harden support for IS in other Sunni areas.

All sides will have to be flexible to make the Mosul battle plan work. Islamic State tactics could involve demolishing whole city blocks as troops move in, or perhaps using the limited chemical weapons IS is believed to have. Few of the Iraqi forces have much training or equipment to deal with such attacks, although that is changing.

Islamic State has done everything it can to manage the information battle within the city. Internet connections have been largely severed, although a variety of options remain for those inside to remain in touch with the outside world. As elsewhere in Syria and Iraq, local journalists and human rights monitors have been deliberately targeted, some executed and others intimidated into silence. The U.S.-backed coalition has conducted leaflet drops and radio broadcasts, although it’s hard to determine their effectiveness.

In Aleppo, many observers suspect Syrian and Russian forces have been deliberately trying to prompt the local population to flee by targeting hospitals and aid deliveries. That seems unthinkable in Mosul, but the overall strategy will still be to persuade the population to flee, risky though it is, so they can be helped by the large humanitarian operation already in preparation outside.

The real danger, though, is that all these plans gradually break down. The longer the battle goes on, the more likely the Iraqi government is to make an already difficult situation even more complex by turning away from Washington to whichever allies or additional forces it can find. These could be Iranian and associated Shi’ite groups or perhaps even Turkey, which now has limited forces in Iraq and is finally itself drawn ever deeper into Syria.

If history is any guide, IS may also respond to the assault on Mosul with terror attacks elsewhere, both within the region and beyond.

Given the stakes, it wouldn’t be surprising if the offensive kept on being postponed, with the militants left running the city while its foes hope the group gradually loses its hold on power. But that seems unlikely. The Iraqi government in particular needs to prove itself too, and that means going in. Whatever the cost.