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 in fact, 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 told The Associated Press.

“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 American troops back from northeastern Syria, paving the way for an assault by Turkey, the Kurds knew exactly 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 into Syria and flush out the Kurdish fighters, whom 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 repeatedly touts as a signature achievement of his presidency.

After all that, would the Americans really abandon them?

Trump sent signals they would, venting regularly about U.S. troops in Syria and wondering why U.S. soldiers were in the Middle East at all. The relationship with the Americans was wobbling.

Sensing an opportunity, Moscow reached out to the Kurds and asked them to forgo their alliance with the United States. Kurdish officials rejected the outreach publicly, saying they were sticking with the Americans.