Christmas came early in Syria. Donald Trump’s surprise tweet heralding the withdrawal of US troops neatly indicated the winners and losers in the murderous eight-year Syrian war. While the US never had much leverage in Syria – thanks to Barack Obama’s disastrous 2013 decision not to act following the Ghouta chemical attacks – Trump has managed, in a 16-word message, to embolden Islamic State, Moscow, Damascus, Hezbollah and Iran. In a sense, he has abandoned any western influence over Syria and handed the territory to dictators, murderers and terrorists.

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First up is the Russian president, Vladimir Putin, who began “engagements” in Syria in 2015 – relentless campaigns that targeted civilians. For Putin, US withdrawal represents a green light to remain in Syria as long as he wishes, to consolidate his power base and pursue his personal Syrian agenda without meddlesome threats from Washington.

For the Syrian president, Bashar al-Assad, it means more time to finish off a war that began as a peaceful demonstration of people calling for their freedom: a war that now involves gulags, unimaginable torture, ethnic cleansing and the chemical gassing of civilians.

In a strange example of camaraderie, turning a blind eye to grievous human rights violations in Syria is one of the few areas where Trump and Obama meet. Still, having the US involved – even minimally – complicated Assad’s scorched-earth campaign against the Syrian opposition. All he needs now is the north-western city of Idlib and the country is his (except that he has to share it with Russia and Iran).

For the Turkish president, Recep Tayyip Erdoğan, who held talks with his Iranian counterpart, Hassan Rouhani, on Thursday, Trump’s message is a blessing. The US involvement in Syria meant empowering the Kurds, whom he has always seen as an existential threat. On Monday, Erdoğan said he was ready to launch a new cross-border military operation at any moment against the Kurdish People’s Protection Units, or YPG, which have been shielded – until now – by the US military presence.

His hawkish stance was echoed by his defence minister, Hulusi Akar, who said Turkey was preparing “intensely” for a military offensive east of the Euphrates River in Syria, where Kurdish-led forces, the Syrian Democratic Front, have battled Isis. The Kurdish fighters there, he told Turkish reporters, have already dug trenches and tunnels in anticipation of a military operation.

“But whatever they dig … when the time comes they will be buried in the trenches,” he gloated. “Of this there should no doubt.”

The Kurds have been betrayed since the collapse of the Ottoman empire, so it’s not entirely surprising for them to be backstabbed. But it is galling. They did most of the heavy lifting in terms of fighting Isis, took many casualties and, in the words of the US general Joseph Votel, were exemplary at “living up to their word”. In short, they were important, essential military partners.

Now they are abandoned. Again. A rapid US withdrawal will leave them vulnerable to the Turks but also will lead to a disintegration of the Arab fighters who were aligned with them in the fight against Isis. Those fighters are now being courted by Assad.

It’s all good news for Iranian militias and Tehran. But the greatest gift of all is to Isis. While Trump boasts that it is finished in Syria, his shortsighted elation over eliminating terrorism is premature. There are still thousands of Isis fighters, they hold a small area of Syria, and sophisticated recruitment continues. If anything, Isis will use Trump’s withdrawal as a powerful recruitment tool.

What Trump has failed to grasp is, while the blows Isis took from the US coalition in Raqqa and Mosul were heavy (and they also inflicted massive collateral damage in both cities), they did not destroy the philosophy that Isis has been able to peddle to disenfranchised Muslims throughout the world. The caliphate was halted – but momentarily. In a disturbing interview this week, Jürgen Stock, the Interpol chief, said that “Isis 2.0” was emerging as a powerful force in Europe, as first-generation fighters will soon be released from prison.

For knowledgeable US officials, the news is devastating – and has caused the US defence secretary, James Matthis, to resign. “Nobody is declaring a mission accomplished,” Brett McGurk, Washington’s top diplomat against Isis, said earlier this month, adding that a long-term campaign to ensure stabilisation was essential. At the Atlantic Council in Washington this week, the US special representative for Syria, James Jeffrey, said: “Isis will come back if the underlying conditions are receptive to that kind of ideological movement.” That time, thanks to Trump, is now.

While the situation for the Kurds is dire, other minorities will also be affected. The Christians, who were targeted, expelled and killed by Isis, whose villages were razed, were beginning the slow process of going home and rebuilding their lives. They are caught between their fear of other rising radical Islamic groups and Iranian militias. They also fear the Kurds. Some members of the Syrian Christian community, for instance, welcome the withdrawal because “Syrian Christians want their country united as one, and the territorial integrity of Syria respected with no separatist regions created in the country,” says Zina Rose Kiryakos, an attorney for Christian victims of Isis and an advocate for Middle Eastern Christians.

She cites reports that the Kurdish-led Syrian Democratic Forces “subjected Christians to persecution and oppression aimed at forcing Christians to leave the area; this includes beatings, arbitrary arrests, closing down Christian schools, attempting to assassinate a Syrian Catholic bishop, and intimidating any Christian who speaks out on what is happening.”

The announcement may be one of Trump’s impulses that change daily, though he is consistent in his distaste for any kind of protracted foreign entanglement. The irony is that he is committing exactly the same blunder he blamed Obama for: withdrawing without stabilising Syria means creating a vacuum that will soon be filled by either Iranian-backed militias or Isis – or by both.

Yet what Trump has done in the long term is far more catastrophic than withdrawing US troops. He has sent a message that will destroy any trust local fighters worldwide will ever have in the US to join forces fighting to eradicate terrorism. He has, in a sense, signed a death warrant for potential military partners and alliances.

We live in a time of conflict fuelled on many levels by terrorism. When the US wants the support of local militias in conflict-ridden areas such as Yemen, Afghanistan, Somalia or the Sahel in fighting terrorists, they will look long and hard at the lesson of the Kurds and what they got from the US. Help us, but at your own peril. We will abandon you whenever we so desire.

• Janine di Giovanni is a senior fellow at Yale University’s Jackson Institute for Global Affairs, and the author, most recently, of The Morning They Came for Us: Dispatches from Syria