Iraqi and Kurdish forces on Monday launched an offensive to take back the city of Mosul from Islamic State (ISIS) fighters, as human rights groups warned of the campaign’s potentially catastrophic impact.

“The hour has come and the moment of great victory is near,” Iraqi Prime Minister Haider al-Abadi said in an early-morning speech, followed by U.S. envoy Brett McGurk tweeting that the U.S.-led coalition was “proud” to support the “historic operation.”

Smoke rose over the city as the U.S.-led coalition began conducting airstrikes shortly after the announcement. The United Nations has long estimated that the battle could displace at least 1 million people, worsening the country’s humanitarian crisis, and the agency’s high commissioner for refugees Filippo Grandi said from Baghdad on Monday that protection of civilians should be “the most important element of this operation.”

But the offensive is rife with diplomatic issues. As the Guardian‘s Jason Burke explains, even if the offensive works to reclaim Mosul, which ISIS took in a 2014 blitz, its long-term impacts may widen sectarian rifts and fuel continued internal conflict—and could change the game in Syria as well. Burke writes:

And in the immediate term, civilians are facing an extreme humanitarian crisis, having reportedly been banned by ISIS from leaving the city and receiving little to no help from the government. Mosul is the militant group’s de facto capital and has a population between 1.2 and 1.5 million people, at least half of which are minors, the charity Save the Children has estimated. The group warned Monday that without civilian protection measures, all of them are at risk.