WASHINGTON — When Syria’s Kurdish fighters, America’s longtime battlefield allies against the Islamic State, announced over the weekend that they were switching sides and joining up with Damascus and Moscow, it seemed like a moment of geopolitical whiplash.

But the move had been in the works for more than a year.

Fearing U.S. abandonment, the Kurds opened a back channel to the Syrian government and the Russians in 2018, and those talks ramped up significantly in recent weeks, American, Kurdish and Russian officials said.

“We warned the Kurds that the Americans will ditch them,” Russia’s ambassador to the European Union, Vladimir Chizhov, told Russia’s Tass news agency on Monday.

The switch in allegiances is a stark illustration of how American foes like Russia and Syria are working steadily to fill the vacuum left by President Donald Trump’s retreat in the region. It also betrays the anxiety that U.S. allies across the globe now feel in the face of Trump’s seemingly impulsive foreign policy decisions, which often come as a surprise to allies and critics alike.

When Trump announced Oct. 6 that he was pulling troops back from northeastern Syria, paving the way for an assault by Turkey, the Kurds knew where to turn.

Syria’s Kurds have publicly acknowledged courting the Syrian government and its allies over the past year.

But much of the back-channel diplomacy, including the most recent talks, happened behind the scenes.

Discussions between the Kurds, the Syrian government and Moscow began early last year as the Kurds grew nervous that the Americans would leave them in the lurch, Kurdish officials said. Pulling U.S. troops out of northeastern Syria would leave the Kurds directly in Turkey’s line of fire, because the Americans served as something of a buffer between the two sides.

The Turks have long been eager for an opportunity to go in and flush out the Kurdish fighters in Syria, which they consider terrorists. Turkey says the group is an offshoot of a Kurdish guerrilla group known as the Kurdistan Workers’ Party, which has waged a decades-long insurgency inside Turkey.

As Turkey spoiled for a fight, the Kurdish fighters were losing confidence in their alliance with the Americans. For five years, the Kurds had fought alongside U.S. soldiers and were vital to defeating the Islamic State group — something Trump touts as a signature achievement of his presidency.

The deal the Kurds’ struck over the weekend with Syria and Russia was negotiated in Aleppo and finalized in Damascus, said Razan Hiddo, senior a Kurdish official. Kurdish forces will work side by side with the Syrian army to try to ward off the Turkish offensive, which began last week after Trump told Turkish President Reccep Tayyip Erdogan that U.S. troops would no longer be in the way.

Russia and Syria, meanwhile, are in a strong position to fill the vacuum left by Trump.

Last week, Russian Foreign Minister Sergey Lavrov was asked about how both the Syrians and the Kurds were looking for Russia to step in as a mediator.

Lavrov made no promises, but said, “We’ll see what we can do.”