WASHINGTON (AP) — Congressional Republicans have spent most of the past two years trying to limit public fights with President Donald Trump, either out of party loyalty or fear of being on the wrong end of a presidential tweetstorm.

But that show of unity was put to the test this week when Trump announced he would withdraw U.S. forces from northern Syria in advance of an impending Turkish military operation against Syrian Kurdish militia fighters.

Senior Republican leaders like senators Lindsey Graham of South Carolina, Mitch McConnell of Kentucky and Ted Cruz of Texas all joined Democratic colleagues in publicly criticizing the idea, with Graham even going on Fox News to label the decision "short-sighted and irresponsible."

But of all the challenges and headaches that the Trump presidency has created for Republican legislators, what made this issue the one that caused them to break ranks?

"It's intense and it's broader than I expected," said Steven Cook, a Middle East specialist at the Council on Foreign Relations. "It's surprising given that we've seen how loyal Republicans have been."

Cook said he believed many of the Republican critics were acting out of genuine concern for the repercussions on the ground.

"I think it's fear of a return of the Islamic State more than anything else," he said.

Personal loyalties toward a staunch U.S. ally and influence from U.S. military leadership have also played a role, Cook said.

"If the Republicans do listen to anybody, it's the people in uniform," said Cook, who said the Pentagon has been "deeply appreciative" of the role played by the Kurdish militia fighters in Syria.

With the help of massive American air support and special forces assistance, Kurdish fighters were the tip of the spear in winning back Islamic State-held territory, including weeks of brutal street-by-street fighting in the final IS strongholds.

"They were the guys on the ground doing a lot of the front-line fighting," and keeping U.S. troops out of harm's way, Cook said. "Thanks to them, U.S. troops didn't have to do some of the heavy lifting. They saved American lives."

Tamara Wittes, a senior fellow at the Brookings Institution's Center for Middle East Policy, said domestic political considerations may have played a role as well. She speculated that some prominent Republicans felt uncomfortable defending Trump over the Ukrainian scandal that prompted a House impeachment inquiry.

"I think it's not just Syria. I think this is about Republicans with a national spotlight feeling the need to show some daylight between them and the president," she said. "Clearly they're feeling some need to show they're not in lockstep."

Wittes also pointed out a frightening potential knock-on effect if Turkey is allowed to attack the Kurds: a mass prison break by Islamic State detainees. Currently militia members are manning a series of prisons in Syria containing thousands of hardcore IS fighters. Those guards would presumably rush to the front lines if their comrades faced the existential threat of the Turkish military machine.

"The only foreign policy issue that Americans really care about is terrorism and the fear of terrorism," she said. "The fear of ISIS 3.0 is very tangible and something that speaks to Americans."