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The battle to liberate Mosul, Iraq's second-most populous city, from the Islamic State of Iraq and the Levant's brutal two-year rule has begun. If successful, ISIL's increasingly tenuous hold on portions of Iraq will soon end.

But however promising, the offensive holds considerable risks, too. More than 1 million citizens are imperiled, and there are dangers to an amalgamation of Iraqi forces, as well as U.S. forces who back them. And winning the peace in a post-ISIL Mosul also would be challenging.

So far, it appears that Iraqi Prime Minister Haider al-Abadi is seeking to minimize the mistakes of his presidential predecessor, Nouri al-Maliki, whose sectarian rule helped create the conditions for the extremist Sunni group to emerge. Kurdish Peshmerga forces began the assault and will hold territory east of Mosul, making way for Iraqi army and special forces troops. Sunni paramilitary forces, as well as some Shia mobilization forces, will take part, too. Military coordination will be essential, but so too will be the political aftermath, lest sectarian divides once again throw Mosul and other predominantly Sunni portions of Iraq into chaos.

While Iraqis are taking the lead, U.S. forces are involved in airstrikes, training and logistics. While nothing near the scale and scope and mission of the 2003 invasion or the years of combat afterward, Americans will be integral to this essential fight, and thus be in harm's way.

"We put this coalition together, we held it together politically, it is our firepower making it hold together militarily," James F. Jeffrey, former ambassador to Iraq and Turkey, told an editorial writer. Jeffrey, now a distinguished fellow at the Washington Institute for Near East Policy, gave context to the campaign by adding, "This is an extraordinary moment in the fight against ISIS. If ISIS loses Mosul in the heart of Sunni Iraq it will essentially mean losing its claim to the caliphate."

A military victory must not become a political defeat. Beyond balancing sectarian dynamics, President Obama, Al-Abadi and other leaders of the multinational anti-ISIL coalition have to act quickly to handle the expected exodus of those Mosul residents who can escape the fighting. This number could be in the hundreds of thousands, and comes amid a region reeling from multiple displacement crises, including Syrian refugees fleeing ISIL, the Assad regime, and other armed groups.

Obama, elected in part as a result of President George W. Bush's multiple Mideast mistakes, has had failures of his own, especially regarding Syrian policy. But his pledge to "degrade and ultimately destroy" ISIL, which sounds like an uncertain trumpet to some critics, has actually been relatively successful, with minimal U.S. casualties.

To be sure, much more needs to be done to defeat ISIL — in Iraq, in its stronghold in Syria as well as in other failed states like Libya, and especially regarding the "cyber caliphate," where the group has metastasized. But for now, Americans should keep in mind the sacrifices of U.S. forces assisting in a fight that Iraq — and the civilized world — must win.