WASHINGTON — American forces were perched high on the top of the Bashiq Mountain, calling in airstrikes that pounded Islamic State militants. Down below, Kurdish forces rolled past to recapture the ancient city of Sinjar.

The December 2015 victory — it also regained control of a strategic roadway linking Iraq and Syria — was a testament to the critical partnership between the U.S. and Kurdish fighters in both Iraq and Syria. It’s an alliance Obama administration officials and even some critics credit with helping diminish the militant group’s aggressive land grab.

But that alliance has been rattled by President Donald Trump’s immigration restrictions and refugee ban. By blocking citizens from several nations in the region from entering the U.S., Trump’s order bars entry for most Kurds, a policy that threatens to estrange some of the United States’ closest allies in the war against Islamic extremism.

Kurds are an ethnic group predominantly concentrated along the borders of four countries — Turkey, Syria, Iraq and Iran. The latter three are among the seven Muslim-majority nations named in Trump’s executive order.

The vast majority of Kurds are Sunni Muslims, a religious minority in Iraq and Iran that has long complained of persecution and alienation by their Shiite-majority governments. While Trump’s order does suggest an exemption “provided that the religion of the individual is a minority religion” in the home country, it does not appear to distinguish between Muslim sects. The order does not specific any exemption for the Kurds or any ethnic group.

“It’s bad, psychologically,” said Bassam Barabandi, political adviser to the Syrian opposition’s High Negotiations Committee. “Do you think the Kurds can live without the Americans? Of course not. But it makes America look anti-Muslim. From that angle, it is for sure dangerous.”

The White House did not respond to a request for comment about how the ban might affect allies in the fight against the Islamic State group.