The capture of ISIS' last, tiny enclave in the country's south brings its self-proclaimed caliphate to territorial defeat.
"Syrian Democratic Forces declare total elimination of so-called caliphate" spokesman Mustafa Bali said on Twitter early Saturday.
Syrian Democratic Forces declare total elimination of so-called caliphate and 0 territorial defeat of ISIS. On this unique day, we commemorate thousands of martyrs whose efforts made the victory possible. #SDFDefeatedISIS
The battle for Baghouz, the group's last holdout and all that remained of the vast territory that it once ruled in Syria and Iraq, had dragged on for more than 10 weeks — far longer than either the U.S. military or their allies on the ground had predicted.
On Friday, the White House said the Department of Defense had declared that the militant group no longer held any territory in Syria. At around the same time, Trump tweeted that there was "nothing to admire" about ISIS.
The militants, meanwhile, had been putting up a desperate fight, and the American-backed Syrian Democratic Forces supported by U.S.-led coalition airstrikes had held off declaring victory.
At a victory ceremony near Baghouz on Saturday, a brass band in red and gold uniforms played the American national anthem in front of a stars and stripes flag and yellow militia banners. SDF leaders including both men and women sat watching.
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"We have won against ISIS. We have beaten them and we have beaten them badly," Trump said in a December video posted on Twitter. "We have taken back the land and now it's time for our troops to come back home."
A recent Pentagon report found the group could reclaim territory in months and is currently regrouping in Iraq faster than in Syria, underscoring the fluid nature of the security situation in the Middle East.
"Currently, ISIS is regenerating key functions and capabilities more quickly in Iraq than in Syria, but absent sustained [counterterrorism] pressure, ISIS could likely resurge in Syria within six to 12 months," according to the Department of Defense Inspector General Quarterly Report about Operation Inherent Resolve.
The U.S. believes Baghdadi, the group's leader, is in Iraq. He stood at the pulpit of the great medieval mosque in Mosul in 2014 to declare himself caliph, sovereign over all Muslims.
The group no longer controls any territory in Syria or Iraq, but continues to carry out insurgent attacks in both countries. It also maintains affiliates in Egypt's Sinai Peninsula, Afghanistan and elsewhere.
Linda Givetash is a London-based freelance journalist.
Ammar Cheikh Omar, Matt Bradley, Reuters and Associated Press contributed.