Karwan Jamal Tahir, the high representative of the Kurdish Regional Government in London, told Newsweek he fears a fresh humanitarian crisis in Syria.
Arizona Rep. Ruben Gallego said the decision to withdraw U.S. troops from the Kurdish-controlled region of Syria was a "tragedy across a huge scale."
"We learnt that President Trump has ordered the U.S. military not to help," Fox News National Security Correspondent Jennifer Griffin said.
"The world will never forgive us. Nor should they. #KurdsBetrayedByTrump," Human Rights Campaign's Charlotte Clymer tweeted.
The president "recognized that sending American troops to Syria is not in the best national security interest of the United States," Andrew Napolitano said.
"Remember that earlier this year, Trump VETOED Congress' attempt to end US involvement in the Yemen War," the Democratic congresswoman tweeted.
Some 2,500 ISIS foreign fighters remain in Kurdish prisons in northeastern Syria, but the Turkish operation may threaten the security of such facilities.
Despite loud protests about American betrayal, core U.S. national security interests in war-ravaged Syria were always narrow and specific: destroy the Islamic State's physical caliphate.
Turkey wants to clear Kurdish armed forces away from its southern border in "Operation Peace Spring."
Allowing Turkey to act against the Kurdish-led SDF in northeastern Syria "will severely damage American credibility," retired General Joseph Votel said.
"The fight against Daesh [ISIS] is not over and continues alongside the [Kurdish-led] Syrian Democratic Forces," Prime Minister Edouard Philippe said.
Just hours after Trump threatened to "obliterate" Turkey's economy, he described the country as one of America's biggest and most honest trading partners
"We are voluntarily abdicating our role and it's the Russians, Iranians and others who are benefitting," Richard Haass warned.
The former U.S. Ambassador to the United Nations said the decision was "appalling" because Kurds had been "promised our partnership."
The president told reporters Monday that an unnamed "top general" lamented the state of U.S. military supplies when he took office.
"I am absolutely appalled that the United States is going to betray those Democratic forces in northern Syria," Pat Robertson said on "The 700 Club."
"By abandoning the Kurds we have sent the most dangerous signal possible – America is an unreliable ally," the Republican senator from South Carolina argued.
"We must always have the backs of our allies, if we expect them to have our back," Nikki Haley tweeted a minute after Hillary Clinton.
The White House has said Turkey will now be responsible for ISIS prisoners held by the Kurds, but an invasion of northern Syria could unleash chaos that ISIS fighters would try to exploit.
"Fox & Friends" host Brian Kilmeade aired his grievances Monday about the president pulling back U.S. troops from Syria alongside Kurdish-led fighters, saying America is "abandoning" its allies.
Donald Trump admitted in 2015 that the towers represent a conflict of interest in his dealings with Turkey.
A former U.S. special envoy to the coalition against ISIS was among many critics of the move, saying that decision to allow Turkey's incursion "guaranteed future bloodshed".
The White House has not clarified what the stay-behind mission will entail, nor how long it will last.
"To those groups who are betting on the Americans, we say the Americans will not protect you," Syrian President Bashar al-Assad said.
The move comes as Kurdish leader Ilham Ahmed's Syrian Democratic Council enters tough negotiations with the Syrian President Bashar al-Assad's government in hopes of finding peace.
The Syrian Democratic Forces claimed President Donald Trump's pullout from Syria was still in its "theoretical form" but they are already reportedly giving up positions in fear of a Turkish attack.
"Nothing can be achieved by threatening Turkey economically," said the country's foreign minister, Mevlut Cavusoglu, as the Trump administration warned Ankara against ordering an offensive against Kurdish forces.