U.S. and allies declare military victory, but is it really the end of the ISIS 'caliphate'?

The announcement of victory over the group Islamic State in Iraq and Syria (ISIS) in eastern Syria marks the end of the extremists' self-styled caliphate, a proto-state in which they held millions hostage to their dark and brutal vision.

But ISIS, which traces its roots back to the bloody emergence of al-Qaeda in Iraq after the 2003 U.S.-led invasion, has survived past defeats and is already waging a low-level insurgency in areas it was driven from months or even years ago.

The gruelling 4-year campaign to drive ISIS from the territories it once held has left entire towns and neighbourhoods in ruins, in both war-torn Syria and Iraq. If the long-standing grievances of Sunni Muslims in both countries continue to fester, the extremists could rise again.

What has ended exactly?

What is over is the militant group's physical "caliphate," after the Syrian Democratic forces, a Kurdish-led group supported by the United States, declared on Saturday the capture of the last tiny patch of territory controlled by the militants at the village of Baghouz, in eastern Syria.

That domain once stretched over large parts of Syria and Iraq, which the group conquered in a blitz in the summer of 2014, capturing towns and cities, including Mosul, Iraq's second-largest. The fighters bulldozed berms along the border and proclaimed a contiguous caliphate stretching across a third of both countries. At its height, the territory was the size of Britain, stretching nearly to the northern Syrian town of Aleppo to the outskirts of the Iraqi capital Baghdad, and home to 8 million people.

The extremists governed under a harsh and violent interpretation of Islam. They massacred those who resisted their rule and beheaded hostages including western journalists and foreign aid workers in gruesome videos circulated online. Alleged adulterers were stoned to death, those believed to be gay were thrown from the tops of buildings, and children were made to watch the atrocities as part of their brainwashing. The group captured thousands of women from Iraq's Yazidi minority, forcing them into sexual slavery.

ISIS also carried out the more mundane actions of a state collecting taxes, printing school textbooks, minting its own currency and restoring public infrastructure. It was an experiment in statehood that not even al-Qaeda ever tried on a significant scale.

From its de facto capital of Raqqa, in northern Syria, its leaders plotted spectacular attacks abroad, including the 2015 Paris attacks that killed 130 people. As ISIS began to hemorrhage territory, it began opportunistically claiming attacks without any evidence of its involvement.

The self-proclaimed caliphate attracted tens of thousands of people from around the world, lured by the group's online activism and slickly produced propaganda videos. Young, troubled men were eager to wage war against those branded enemies of Islam, while others were drawn to the promise of life in an Islamic state governed by God's law.

That physical "caliphate" was declared dead, for now.

What is the cost of liberation?

The gruelling four-year air and ground campaign against ISIS has killed or wounded tens of thousands of people, drove hundreds of thousands from their homes and left a swath of destruction stretching from the suburbs of Damascus to central Iraq.

The major cities ISIS once held Mosul, Raqqa, Fallujah and Ramadi have all seen major devastation.

The group put up fierce resistance nearly everywhere, using civilians as human shields and launching waves of car bomb and suicide attacks. As it slowly retreated, it left behind booby-traps and explosives that in many areas have yet to be cleared.

The U.S.-led coalition dropped tens of thousands of bombs over Syria and Iraq to help its allies on the ground advance, sometimes pulverizing entire city blocks. Syrian government forces backed by Russian air power battled ISIS in some areas, as did Iraq's state-sanctioned militias, with help from Iran.

The death toll from the campaign remains uncounted. In a report released last year, the coalition confirmed the deaths of 1,139 civilians in airstrikes conducted between August 2014 and November 2018. Rights groups say the number is much higher.

An Associated Press investigation found at least 9,000 civilians died in the assault to retake Mosul alone. In Raqqa, the U.S.-backed campaign killed hundreds of civilians and caused destruction on a massive scale.

Syria is still mired in civil war, and Iraq estimates it will need $100 billion US to rebuild. Local leaders in Mosul say they need that much for their city alone. No one has offered to foot the bill, and hard-hit areas remain empty, even years later.

What's next?

The official declaration of victory is of mostly symbolic value. Thousands of ISIS militants have dispersed and gone to ground, and U.S. defence officials have warned that ISIS could stage a comeback in Syria within a year if military and counter-terrorism pressure is eased.

"They've cut the trunk of this malignant tree, but they haven't pulled up its roots, which are still capable of growing and spreading," Hisham al-Hashemi, a researcher in extremism and expert on ISIS, wrote in a Twitter post.

Activists who closely follow the conflict in Syria already point to signs of a growing insurgency and sleeper cells carrying out assassinations, setting up flying checkpoints and claiming roadside bombs in liberated areas across Syria and Iraq.

They've cut the trunk of this malignant tree, but they haven't pulled up its roots, which are still capable of growing and spreading.- Researcher Hisham al-Hashemi

That insurgency could gain strength as U.S. President Donald Trump presses ahead with his planned U.S. withdrawal from Syria. The American commander overseeing the fight against ISIS, Gen. Joseph Votel, has warned that the group is far from being defeated, saying its leaders have dispersed and gone underground.

"What we are seeing now is not the surrender of ISIS as an organization but a calculated decision to preserve the safety of their families and preservation of their capabilities," he said earlier this month, adding that the insurgents are "waiting for the right time to resurge."

The withdrawal of American forces from eastern Syria would open the door for major turmoil as various actors including the Syrian government, allied with Russia and Iran race to fill the vacuum.

ISIS was all but defeated once before, when U.S. forces withdrew from Iraq in 2011; experts warn it could stage another devastating comeback.

And ISIS has established affiliates across Asia and Africa, and continues to be active in places like Afghanistan, Libya, Egypt's Sinai Peninsula, Yemen and the Philippines.

What about detained fighters and their families?

Another major concern is jihadis finding their way back to Europe.

Around 1,000 foreign fighters are currently being held in Kurdish-run prisons in northern Syria. Their wives many of them from Western countries and their children are in camps in northern Syria.

Syrian Kurdish authorities are calling on countries to take back their nationals, saying they cannot afford to keep shouldering the burden. Trump has weighed in, calling on Britain, France, Germany and other European allies to repatriate their nationals and put them on trial.

"The U.S. does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go," he tweeted in February.

But few countries are willing to bring back people they view as a security threat, posing a dilemma for the Kurdish-led forces as the U.S. prepares to withdraw.