The Islamic State’s capital in Syria fell to U.S.-backed forces Tuesday, the most significant defeat for the militant group since it burst onto the world stage three years ago as a seemingly invincible force.

The defeat of the Islamic State, or ISIS, in Raqqa after a four-month battle with U.S.- backed forces leaves only remnants of the group along the Euphrates River Valley stretching between Iraq and Syria.

ISIS fighters have been pushed out of most of their major strongholds in both countries, bringing to a crashing end the group's ambitious vow to create a powerful "caliphate" it would rule across the Middle East.

The announcement of Raqqa's liberation was made by the Syrian Democratic Forces, the coalition of Kurdish and Arab fighters backed by U.S. air power, advisers and weapons.

Col. Ryan Dillon, a coalition military spokesman, said more than 90% of Raqqa has been cleared, and about 100 militants remain inside the city. Pockets of resistance and booby traps still need to be cleared.

What was supposed to be a cataclysmic battle ended relatively quickly as exhausted militants in the northern Syrian city surrendered, attempted to flee or were killed by coalition airstrikes and ground attacks.

The defeat in Raqqa doesn’t spell the end of the Islamic State. ISIS is already shifting its focus to bombings, targeted assassinations and other terror tactics, said Seth Jones, an analyst at the RAND Corp., a think tank.

But the collapse of the Islamic State's holdings in Iraq and Syria suggests its leadership overreached in attempting to grab territory that it couldn't hold, dealing a significant blow to the terror group's image.

The leader of ISIS, Abu Bakr al-Baghdadi, had announced the establishment of the caliphate in 2014 from a mosque in Mosul, the second-largest city in Iraq. Militants were pushed out of Mosul by Iraqi security forces this year after a nine-month battle.

The ability to govern towns and cities made ISIS different than most other militant groups, but it also allowed the U.S.-backed coalition to target it. U.S.-led coalition aircraft bombed the terror group's warehouses of stolen cash, oil facilities, heavy weapons and armored vehicles and tanks.

ISIS used oil sales, plundered loot and relied on extortion to fund city governments, which provided basic services but imposed a strict version of observing Muslim customs and dealt brutally with opponents of its rule. ISIS regularly used civilians as "human shields."

At its peak in 2014 and 2015, ISIS controlled large swaths of territory in Iraq and Syria, ruling over about 11 million people, according to RAND Corp.

The number of ISIS fighters in Iraq and Syria is now estimated at about 6,500, down from a peak of nearly 30,000, according to the Pentagon.

Groups like the Islamic State and al-Qaeda have proved resilient. They have found refuge in countries plagued by civil war or weak central governments to establish new strongholds, including in Africa, Afghanistan and elsewhere in the Middle East.

"We're all paying close attention to whatever ISIS transforms into next," Army Col. Pat Work, a U.S. adviser in Iraq, said in an interview several weeks ago.

But the collapse of the caliphate will hurt ISIS' global recruitment and tarnish the terror group's image as an invincible force with grand ambitions.

"It does not want to be an insurgency," Work said. "It wants to hold terrain, dominate populations, administer its deformed politics, and it wants to be able to expand continuously."

The collapse of ISIS territory demonstrates the effectiveness of the U.S. strategy to support local forces rather than deploy thousands of conventional forces, said Jones, the RAND analyst.

The U.S. has about 500 troops in Syria and about 5,000 in Iraq. Their mission is to support and advise local forces battling the ISIS. The troops generally are not involved in direct combat.

By contrast, the United States at its peak had about 100,000 combat troops in Afghanistan and more than that in Iraq.

"This is a much more effective model for fighting terrorist groups than sending in conventional forces," he said.

More:1,000 ISIS militants surrender as Iraq retakes key town of Hawija

More:U.S. coalition slashes ISIS oil revenue by more than 90%