International military observers were quick to denounce the president’s triumphalism as premature or naive and, a day later, secretary of defence Jim Mattis resigned, telling Mr Trump in his farewell letter the position should be held by someone “whose views are better aligned with yours”.

Pentagon chief of staff Kevin Sweeney and Washington envoy Brett McGurk also resigned over the issue.

National security advisor John Bolton has since visited the Middle East to reassure the US’s allies in the Syrian Democratic Forces (SDF) they will be shielded from Turkish aggression, incurring the wrath of Recep Tayyip Erdogan in Ankara.

Turkey’s president considers one of the SDF’s constituent militia groups, the Syrian Kurdish People’s Protection Units (known as YPG), a terrorist organisation.

With the US caught between conflicting loyalties to the SDF and Turkey, which is a fellow Nato member, the truth on troop withdrawal, as well as the future threat posed by Isis, is a great deal less clear-cut than Mr Trump suggests.

What the president says

“We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency”, Donald Trump tweeted on 19 December.

A day later, obviously bruised by the scepticism with which his remarks were greeted, Mr Trump repeated praise from Fox News pundit Laura Ingraham and became defensive, making clear his priorities were domestic, not foreign.

“Getting out of Syria was no surprise. I’ve been campaigning on it for years, and six months ago, when I very publicly wanted to do it, I agreed to stay longer. Russia, Iran, Syria & others are the local enemy of ISIS. We were doing there work. Time to come home & rebuild. #MAGA,” he wrote.

His distaste for the fight was confirmed by remarks made during a televised cabinet meeting on 2 January, in which Mr Trump dismissed Syria as an arena of “sand and death”.

“We’re talking about sand and death. That’s what we’re talking about. We’re not talking about vast wealth. We’re talking about sand and death,” he said.

There is still, however, no timeline in place for American withdrawal at the time of writing.

The situation on the ground

President Trump was emboldened to hail the end of Isis after the SDF announced it had taken back most of the town of Hajin on 14 December.

Lying on the banks of the Euphrates, Hajin had been the scene of bloody fighting since September between US soldiers, the SDF and the last of the Islamist terrorist fighters, already driven out from the north and east of the country.

The town represented a last stand for many of the extremists' most experienced fighters and the battle left heavy casualties on both sides.

The UK-based Syrian Observatory for Human Rights estimated in December that some 793 Isis fighters and 464 SDF soldiers had been killed in the three-month assault on Hajin. There will also significant civilian casualties, many swept up in allied air strikes.

The surviving 5,000 Isis combatants had been driven back to Deir Ezzor, a final outpost the SDF expected to capture in the “coming weeks”, according to commander Lilwa al-Abdullah.

If this does come to pass, a military conquest may not mark the end of the feared terrorist organisation.

As James Jeffrey, the US special representative for Syria, observed in November: “An enduring defeat means not simply smashing the last of Isis’s conventional military units holding terrain, but ensuring that Isis doesn’t immediately come back in sleeper cells, come back as an insurgent movement.”

Republican senator Lindsey Graham agreed, branding the removal of US forces “a mistake” in response to the president’s tweet.

“An American withdrawal at this time would be a big win for Isis, Iran, Bashar al-Assad of Syria and Russia. I fear it will lead to devastating consequences for our nation, the region, and throughout the world,” he said in a statement.

“It will make it more difficult to recruit future partners willing to confront radical Islam. It will also be seen by Iran and other bad actors as a sign of American weakness in the efforts to contain Iranian expansion.”

French president Emmanuel Macron was also critical: “I very deeply regret the decision made on Syria. To be allies is to fight shoulder to shoulder, it’s the most important thing for a head of state and head of military. An ally should be dependable.”

The threat from Turkey

With the resignation of General Mattis from the Pentagon, a development President Trump characteristically sought to rebrand as “essentially a firing”, Mr Bolton has assumed responsibility for Syria.

“There are objectives that we want to accomplish that condition the withdrawal,” he said in Israel on 6 January, alluding to the America’s allies in the YPG when he stressed the president “will not allow Turkey to kill the Kurds”.

Mr Bolton said the chairman of the joint chiefs of staff, General Joseph Dunford, would continue negotiations with his Turkish counterparts to seek protection for America’s Kurdish allies in Syria.

He also said US soldiers would remain in al-Tanf in southern Syria to counter growing Iranian activity in the region, all of which placed him at odds with Mr Trump – and President Erdogan.

“It is not possible for us to accept and swallow the message that Bolton gave from Israel,” Mr Erdogan told his country’s parliament in a televised speech on 8 January, after apparently refusing to meet with the hard-line US hawk.

“If the US evaluates them [the YPG] as ‘Kurdish brothers’ then they are in a serious delusion,” the Turkish president said. “John Bolton made a serious mistake on this issue. Whoever thinks like that is making a mistake.”

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President Erdogan also criticised the US military’s aerial bombardment of the cities of Raqqa and Mosul and repeated his threat “to neutralise those terror organisations in Syria very soon”.

He has made similar threats before and in March 2018 sent his army and allied Syrian rebel groups to take majority control of the Kurdish province of Afrin from YPG, the authoritarian Turkish leader fearing an empowered Kurdish nationalist movement on his doorstep.

The Kurds have meanwhile begun talks with the Assad regime as well as Vladimir Putin’s government in Moscow to handover border areas to Syrian forces in return for a guarantee of internal autonomy over the Kurdish region of Rojava, pointing out they have no alternative in the wake of Mr Trump’s decision.

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“If there is a Turkish attack, the YPG forces will be forced to come protect the borders, to defend their families,” YPG general Mazloum Kobani warned on 13 December.

A US withdrawal from the region risks leaving its former SDF allies exposed to an attack from Turkey, which, if it were to happen, would give the last remaining remnants of Isis a chance to regroup, potentially posing a revitalised terror threat in a new guise.