U.S.-backed fighters have launched an operation to try to seize control of the city of Raqqa in northern Syria, ISIS’ so-called “capital.”

Raqqa is the most important stronghold for the militant group in Syria and fell under its control in 2014. And together with the ongoing fight for Mosul in Iraq, seizing control of the city is seen as a crucial goal in the fight against ISIS.

“The International Coalition and our partner forces are steadily dismantling the physical caliphate of ISIS,” said Lt. Gen. Steve Townsend, the commanding general of Operation Inherent Resolve, which is targeting ISIS in Iraq and Syria.

The Syrian Democratic Forces and their Syrian Arab Coalition partners began the operation Tuesday, according to a statement from Operation Inherent Resolve.

“We declare today the beginning of the great battle to liberate the city of Raqqa, the alleged capital of terrorism and terrorists,” SDF spokesman Talal Sillo told a news conference, The Associated Press reported. “Morale is high and military readiness to implement the military plan is complete, in coordination with the U.S.-led coalition.”

The SDF says it will “attack from the north, east and west” of Raqqa, as NPR’s Alison Meuse reports from Beirut.

Since November, the U.S.-backed troops have been attempting to isolate the city in preparation for the start of the operation to actually take control of it. The offensive is backed by U.S.-led air support and military advisers on the ground. A U.S. Marine artillery unit is positioned north of Raqqa, as NPR’s Tom Bowman has reported.

Townsend stressed that the fight will be “long and difficult” but could be a major turning point in the fight against ISIS. “It’s hard to convince new recruits that ISIS is a winning cause when they have just lost their twin ‘capitals’ in both Iraq and Syria,” he said.

As Tom noted, there are also thousands of civilians who are still in the city and observers are concerned that they will be used as human shields.

After three years, ISIS is firmly entrenched in Raqqa. It has circled the city with land mines and checkpoints, the AP reported, in addition to “setting up barriers and hanging sheets of cloth over main streets to provide cover from warplanes.”

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