NEW YORK (NYTIMES) - United States forces and their Kurdish-led partners in Syria had been conducting as many as a dozen counter-terrorism missions a day against Islamic State in Iraq and Syria (ISIS) militants, officials said.

That has stopped.

Those same partners, the Syrian Democratic Forces, had also been quietly releasing some ISIS prisoners and incorporating them into their ranks, in part as a way to keep them under watch. That, too, is now in jeopardy.

And across Syria's porous border with Iraq, ISIS fighters are conducting a campaign of assassination against local village headmen, in part to intimidate government informants.

When President Donald Trump announced this month that he would pull US troops out of northern Syria and make way for a Turkish attack on the Kurds, Washington's one-time allies, many warned that he was removing the spearhead of the campaign to defeat ISIS, also known as Islamic State.

Now, analysts say that Mr Trump's pullout has handed ISIS its biggest win in more than four years and greatly improved its prospects.

With US forces rushing for the exit, in fact, US officials said last week that they were already losing their ability to collect critical intelligence about the group's operations on the ground.

"There is no question that ISIS is one of the big winners in what is happening in Syria," said Dr Lina Khatib, director of the Middle East and North Africa Programme at Chatham House, a research centre in London.

Cutting support for the SDF has crippled the ability of the US and its former partners to hunt down the group's remnants.

News of the US withdrawal set off jubilation among ISIS supporters on social media and encrypted chat networks. It has lifted the morale of fighters in affiliates as far away as Libya and Nigeria.

And, by removing a critical counterforce, the pullout has eased the re-emergence of ISIS' core as a terrorist network or a more conventional, and potentially long-lasting, insurgency based in Syria and Iraq.

Although Mr Trump has repeatedly declared victory over the ISIS group - even boasting to congressional leaders last week that he had personally "captured ISIS" - it remains a threat.

After the loss in March of the last patch of the territory it once held across Syria and Iraq, ISIS dispersed its supporters and fighters to blend in with the larger population or to hide out in remote deserts and mountains.

The group retains as many as 18,000 "members" in Iraq and Syria, including up to 3,000 foreigners, according to estimates cited in a recent Pentagon report. Abu Bakr al-Baghdadi, ISIS' self-proclaimed caliph, is still at large.

"Our battle today is one of attrition and stretching the enemy," Al-Baghdadi declared in a video message released in April.

Looking comfortable and well fed, he sat on the floor of a bare room, surrounded by fighters, with an assault rifle by his side.

"Jihad is ongoing until the day of judgment," he told his supporters, according to a transcript provided by SITE Intelligence Group.

Against the benchmark of ISIS's former grip on a broad swathe of geography, any possibility of a comeback to that extent remains highly remote.

Changes in the political context in Syria and Iraq have diminished ISIS' ability to whip up sectarian animosity out of the frustrations of Sunni Muslims over the Shi'ite or Shi'ite-linked authorities in Syria and Iraq - the militants' trademark.

The government in Baghdad has broadened its support among Sunni Iraqis. President Bashar al-Assad of Syria, by crushing the revolt against him, has left Sunni militants less space to mobilise. And many Syrians and Iraqis who lived under the harsh dominion of ISIS strongly oppose its return.

But as an underground insurgency, ISIS appears to be on the upswing.

Militants have been carrying out "assassinations, suicide attacks, abductions, and arson of crops in both Iraq and Syria", according to a report this summer by the Pentagon inspector-general for operations against the ISIS group.

It is establishing "resurgent cells" in Syria, the report said, and "seeking to expand its command and control nodes in Iraq".

The militants have been burning crops and emptying out whole villages. They have been raising money by carrying out kidnappings for ransom and extorting "taxes" from local officials, often skimming a cut of rebuilding contracts.

Their attacks on village headmen - at least 30 were killed in Iraq in 2018, according to the Pentagon report - are an apparent attempt to scare others out of cooperating with Baghdad.

"The high operational tempo with multiple attacks taking place over a wide area" may be intended to create the appearance that ISIS can strike anywhere with "impunity", the report said.

Mr Trump first said in December 2018 that he intended to withdraw the last 2,000 US troops from Syria; the Pentagon scaled that back, pulling out about half of those troops.

Military officials, though, say that helping the SDF hunt down underground cells and fugitive fighters required more training and intelligence support than an open battle for territory.

Even the partial drawdown, the Pentagon inspector-general's report found, could be "detrimental" to the US mission in Iraq and Syria.

Last month, as if to prove its continued vitality, ISIS claimed responsibility for a minibus bombing that killed a dozen people near the entrance to a Shi'ite pilgrimage site in the Iraqi city of Karbala. It was its deadliest attack since the loss of its last territory.

And within hours of Mr Trump's announcement almost two weeks ago that US forces were moving away from the Syrian border with Turkey, two ISIS suicide bombers attacked a base of the SDF in the Syrian city of Raqqa.

"The crusaders have given up," ISIS supporters crowed, according to Mr Laith Alkhouri of the business risk consulting company Flashpoint Global Partners, who monitors the group's online messages.

Other messages "urged ISIS 'soldiers' everywhere to double their efforts", Mr Alkhouri said.

The missions against ISIS conducted by the SDF - sometimes as many as two dozen a day - had included both counter-terrorism patrols and raids on militant cells.

Some were carried out jointly with US soldiers, others alone, according to US officials.

But the Kurds, an ethnic minority sometimes disparaged by Arab Syrians, faced resentment among the Arab residents of north-eastern Syria.

In part to try to win support from those communities, the Kurdish-led forces pardoned and released hundreds of detained ISIS fighters or supporters in so-called reconciliation deals, relying on informal relationships with community leaders to handle their reintegration.

The Kurdish-led militia even incorporated some of the released ISIS detainees into its own forces, said Ms Dareen Khalifa, a researcher with the International Crisis Group who has travelled to the region extensively and documented the "reconciliation" pardons in a report last summer.

She said in an interview that the Kurdish militia leaders said: "What do you want us to do, kill them all? Imprison them all? The best way forward is to keep a close eye on them by keeping them within the SDF."

She said that those enlisted had not been ISIS leaders and that so far there had been no recidivism.

But now the US withdrawal and the Turkish incursion are threatening the informal supervision of those former prisoners, Ms Khalifa said, creating a risk that some might gravitate back to fighting for the ISIS group.

Turkey, which has battled Kurdish separatist militants at home for decades, launched the invasion primarily to push back the Kurdish-led forces in Syria. Without US protection, the Kurdish leaders are now switching sides to ally with Mr Assad.

In Iraq, too, some say opportunities may be emerging for ISIS to revive its appeals to Sunni resentments in the areas it once controlled.

Promises of post-war reconstruction have gone unfilled. And Shi'ite militias that rose up to defeat ISIS remain in place, sometimes seeking to profit off the local populations.

"People in the liberated areas say: 'Why are all these armed groups still around? Why do they still call us all ISIS, and why are they taxing us or extorting us and taking all of our money?'" said Dr Renad Mansour, director of the Iraq Initiative at Chatham House.

The campaign against ISIS, he said, "was a military solution to what is a social and political problem".

Mr Trump, for his part, has insisted repeatedly that Turkey should take over the fight against ISIS in Syria.

"It's going to be your responsibility," Mr Trump said he told the country's President, Mr Recep Tayyip Erdogan.

But current and former US officials say the Turkish military has a bleak track record at counter-terrorism and little hope of filling the void left by the Americans and the Syrian Democratic Forces.

"That is wishful thinking as far as I can tell," said Ms Dana Stroul, co-chairman of the congressionally sponsored Syria Study Group and a former Pentagon official.