WASHINGTON (AFP) - Now that the Islamic State in Iraq and Syria (ISIS) has officially lost its geographic "caliphate", the Pentagon is marking a historic moment in its years-long campaign to defeat the militants.

From a military perspective at least, the United States can claim significant success in its strategy of working "by, with and through" local proxy forces, where a Kurdish militia in Syria and security forces in Iraq bore the brunt of the fighting - and dying.

But ISIS still has thousands of battle-hardened fighters across several countries, and questions loom over whether the group's territorial loss can be parlayed into an enduring defeat - or whether President Donald Trump's decision to pull most troops from Syria is premature, and risks ruining the end game.

"I'd be hesitant to use the term winning," General Raymond Thomas, who heads US Special Operations Command, told lawmakers recently.

The objective is "to be able to maintain persistent capabilities so that an external threat cannot emanate from that in the future". Asked if he was satisfied the US was at that point, Gen Thomas said: "I do not think we're there yet."

How much the US can influence things will only diminish after the Pentagon withdraws all but 200 of the 2,000 or so special forces from Syria that have been helping the Kurdish-led Syrian Democratic Forces.

Mr Trump in December declared victory over ISIS, saying the US had "beaten them badly" as he announced the pullout.

Mr John Spencer, a scholar at the Modern War Institute at West Point, said things were not so simple.

ISIS "is a terrorist organisation, all they have to do is put down their weapons and try to blend in with the population and just escape," he told AFP.

"They're not gone, and they're not going to be gone," he said.

STARTED UNDER OBAMA

The US-led mission began in late 2014 under president Barack Obama, after ISIS fighters seized an area the size of Britain across Iraq and Syria.

In an effort to "degrade and ultimately defeat" the black-flag-flying hardliners, the US formed a coalition that grew to more than 70 nations, several of which started bombing ISIS positions in 2014.

In the years since, the coalition has conducted about 34,000 air strikes in Syria and Iraq.

Instead of committing large numbers of troops, the coalition combined its air campaign with training and advising to local forces.

The decision stemmed partly from the Iraq War, which saw more than 4,400 US troops die.

An American public wary of additional deployments did not want Mr Obama recommitting more combat troops.

The strategy paid off fastest in Iraq, where a national military that had neared collapse in the face of the ISIS advance morphed into an army that ousted the militants from one city after another until retaking their stronghold of Mosul in 2017.

When Mr Trump took office, he essentially continued Mr Obama's strategy, albeit with tougher talk and looser constraints on air strikes.

"Overall, the US strategy was effective at pushing back the Islamic State," Dr Daniel Byman, a senior Fellow at the Centre for Middle East Policy at the Brookings Institution, told AFP.

However, he noted, it has not solved the problem of local governance in Syria, where the grueling civil war gave rise to the conditions that allowed ISIS to blossom in the first place.

"So the Islamic State is remaining active - hundreds of killings this month alone - as an insurgency," Dr Byman said.

The toll on US-backed local forces has been brutal, with thousands of Syrian and Iraqi fighters killed.

'IT'S JUST NO'

Mr Trump's decision to withdraw from Syria has left Kurdish partners scrambling for safeguards, and they are hoping a "safe zone" in the north can provide them cover.

A US departure makes Kurdish fighters more vulnerable to attack by neighbouring Turkey, which considers them to be "terrorists", and dashes their dreams of autonomy.

The New York-based Soufan Group, which compiles security assessments, cautioned against claims of beating ISIS.

That would "only serve to offer a false sense of security while showing that the United States remains out of touch with realities on the ground", Soufan said.

General Joseph Votel, who heads the US military's Central Command, said the military cannot take its eye off ISIS.

"The coalition's hard-won battlefield gains can only be secured by maintaining a vigilant offensive against a now largely disbursed and disaggregated (ISIS) that retains leaders, fighters, facilitators, resources and the profane ideology that fuels their efforts," said Gen Votel, who is about to retire, adding that Mr Trump never checked in with him about a Syria withdrawal.

Acting Defence Secretary Patrick Shanahan has tried to convince skeptical allies to help secure Syria.

But "it is totally out of the question to have French troops on the ground without the Americans there", one French government source told AFP.

"It's just no."