Sen. John Cornyn of Texas defended President Trump's widely panned decision to abandon U.S.-allied Kurdish forces in northern Syria.
The Kremlin also said the United States had been the closest ally of the Kurdish fighters but had now betrayed them.
The agreement shows how Trump's abandonment of the Kurds is making the U.S. less relevant in a key region.
Both sides have accused each other of repeatedly violating the U.S.-brokered cease-fire.
Pulling out of Syria is a "grave strategic mistake," the Senate majority leader warned in an op-ed.
"It's the responsibility of the people that show up at those rallies to not be stupid, to not be so stupid that they should be kept away from blenders."
Vice President Mike Pence trumpeted the deal as a "cease-fire," but Turkey disputed that characterization.
White evangelicals have stuck by Trump through allegations of sexual assault, racism and corruption. But leaders have decried his decision to withdraw troops from northern Syria.
The bestselling horror author thinks this is how historians will look back on the era.
The former defense secretary warned that national security is "tied inextricably to our alliances," following Trump's decision to abandon the Syrian Kurds.
But President Trump says it’s OK for Turkey and the U.S.-allied Kurds to fight. "Let them!" he tweeted.
A U.S. military official said the situation across northeastern Syria was "deteriorating rapidly."
Kurds have come to deeply value their collective memory. The next generation won't forget U.S. support for Turkey's bloody move into Syria.
Turkey’s continued push into Syria comes days after President Donald Trump cleared the way for Turkey’s air and ground offensive.
"Pull my name off the 'I support Donald Trump' list," Shimkus said.
The president warned he will take action should the country do anything that he considers "off limits" in his "great and unmatched wisdom."
Sen. Lindsey Graham of South Carolina called it "a stain on America's honor."