The Islamic State was routed Monday from one of its key strongholds on Syria’s border with Turkey after its defenses crumbled and its fighters either defected or fled, raising new questions about the group’s vaunted military capabilities.

The fall of the town of Tal Abyad to a Kurdish-Syrian rebel force backed by U.S. airstrikes came after just two days of fighting during which the militants appeared to put up little resistance, focusing instead on escaping to their nearby self-styled capital of Raqqa or fleeing across the border to Turkey.

The force — led by Kurdish units of the People’s Protection Units, or YPG, and including local battalions of the rebel Free Syrian Army — pulled the Islamic State flag down from the border crossing with Turkey on Monday and by nightfall said it was in control of the town center.

There were reports of scattered fighting on the western outskirts of Tal Abyad, but the advancing force had already severed the militants’ escape route, closing in on the town Sunday in a pincer movement from the east, south and west.

It appeared the Islamic State had suffered a stunning defeat, its first major reversal since it was driven out of the Iraqi city of Tikrit in April, and one that could prove far more consequential. Tal Abyad commands the major trade and smuggling routes on which the Islamic State has relied for its supplies from the outside world and, most significant, the flow of foreign fighters to Raqqa, the first major city it conquered.

If the Kurdish-led force consolidates its hold over Tal Abyad, “it will be a major setback for the Islamic State and a major strategic victory,” said Jennifer Cafarella of the Washington-based Institute for the Study of War.

The militants may be planning a counteroffensive, “and it will be interesting to see what they do in the coming week,” she added.

But there were also signs that at least some Islamic State fighters had simply given up.

Photographs posted on social media by local activists showed groups of Islamic State fighters surrendering to Turkish forces and being led away after fleeing across the border. Syrians who had fought with the Islamic
State were among more than 10,000 refugees who scrambled across the border in recent days to escape the battles, according to a Syrian aid worker who said he had been contacted by several seeking assistance.

More than 500 local residents who had joined the Islamic State defected to the advancing force, which includes Syrian rebel units driven out of the area after losing battles to the militants in 2013, he said.

The speed of the collapse was unexpected and suggests that the Islamic State has shifted its tactics since its ill-fated assault last September on the far less significant border town of Kobane, which it had been poised to overrun until the U.S.-led coalition created to confront the Islamic State intervened with airstrikes.

Even after it had become clear that its efforts to control Kobane had failed, the Islamic State stood its ground, clinging to its positions around the town, enduring wave after wave of airstrikes and suffering what U.S. and Kurdish officials say were thousands of casualties.

In the case of Tal Abyad, the Islamic State appears to have chosen to regroup rather than stand and fight. Convoys of fighters headed south to Raqqa over the weekend, some of them carrying medical equipment from Tal ­Abyad’s hospitals and stocks of flour from its silos, according to the activist network Raqqa Is Being Slaughtered Silently.

On Monday, the militants were digging trenches to the north of their capital in a sign that they are planning to entrench there, the network reported.

The battle for Tal Abyad represented an extension of the fight for Kobane, with Kurdish forces continuing to press east after they ejected the militants from their town in January. They have since linked up with more Free Syrian Army battalions as they pressed deep into the Islamic State’s home turf of Raqqa, uniting under a coalition known as Burkan al-Furat, or Euphrates Volcano.

There are concerns, however, that the alliance between the Kurdish YPG and the Syrian Arab rebel units may fray now that they are conquering territories with a majority-Arab population.

The capture of Tal Abyad represents a major boost for the region’s Kurds, giving them control of a contiguous stretch of territory from the Iranian border with Iraq to the heart of Syria.