Early Monday morning, Iraqi Prime Minister Haider al-Abadi went on live TV to announce the launch of a military campaign to retake the city of Fallujah, which has been controlled by ISIS since early 2014.

“Today, we will tear down the black flags of these despicable strangers who abducted this dear city,” said al-Abadi. “The time has come to liberate Fallujah and the victory will be ours.”

Citing an unnamed security official, NBC News reports 20,000 federal troops have now been sent to the key ISIS stronghold with 15,000 more on their way. From The Washington Post:

Few expect an easy fight. Islamic State militants have dug in and built defenses in the city since capturing it more than two years ago, the first in the country to fall to the extremist group. Fallujah has long been considered a hotbed of rebellion and extremism, with even the heavy-handed Saddam Hussein struggling to control its tribes. U.S. Marines fought Sunni insurgents during two battles for the city in 2004, the second of which marked the heaviest urban combat for U.S. troops since the Vietnam War, killing nearly 100 service members.

It’s not an order of battle that correlates to U.S. military policy, which had focused on an offensive targeting Mosul, the Islamic State-held city farther north. President Obama has said he expects the recapture of Mosul to be close to complete by the end of the year. But a drawn-out battle for Fallujah could delay the already sputtering buildup to that offensive.

On Sunday, Iraqi security forces urged civilians to flee the city once home to 300,000, but residents say that extremist-controlled checkpoints have made escape impossible for most, the Associated Press reports.

According to CNN, thousands of leaflets promising safe passage for civilians have been dropped on the city by Iraqi planes. Those who cannot leave were asked to fly white flags over their homes.