Turkish troops and aircraft launched their long-awaited assault into Syria Wednesday after Trump ordered US troops in the area pulled back, leaving the Kurds -- whom Ankara calls a terror threat -- fully exposed.
Former Middle East commander Joseph Votel has accused Trump of giving up longstanding allies who, as the core of the Syrian Democratic Forces, were crucial in the half-decade campaign to crush the Islamic State group.
"This policy abandonment threatens to undo five years' worth of fighting against ISIS and will severely damage American credibility and reliability in any future fights where we need strong allies," Votel, who retired earlier this year, wrote in The Atlantic.
"The SDF freed tens of thousands of square miles and millions of people from the grip of ISIS. Throughout the fight, it sustained nearly 11,000 casualties," he said.
By comparison, he noted, only six US troops and two American civilians were killed in that campaign.
Trump: US helped Kurds enough
Trump has said he wants to end the US involvement in Syria and also explained that the US cannot do anything about a centuries-old hate between the Turks and Kurds.
He suggested that the United States has aided the Kurds enough.
"They are fighting for their land," he told reporters.
"They didn't help us with the Second World War. They didn't help us with Normandy, as an example.... We have spent tremendous amounts of money on helping the Kurds in terms of ammunition, in terms of weapons, in terms of money."
"With all of that being said, we like the Kurds," he added.
Mark Hertling, the former commanding general of the US Army's European operations, said Trump's decision "presages an upcoming disaster for the US."
"Kurds of the SDF -- our former reliable partners in the fight against ISIS -- are being attacked by a NATO ally, Turkey. The repercussions for the US and NATO will be long-lasting and detrimental to security for Europe and the world," he said on Twitter.
The SDF took the lead in the major campaigns to dislodge Islamic State fighters from key cities in brutal sieges that they led.
Pentagon officials said they were better trained and focused to do the job compared to Turkish and Iraqi troops and other possible proxies in the war.
"When the Iraq army collapsed, the Kurds were the ones who took the full brunt of ISIS' attack on civilization, not us, not the Turks," said Ruben Gallego, an Iraq war veteran and Democratic congressman.
Parallels in US past
Some are drawing a parallel to previous episodes in US wars abroad when an ally was left to fend for itself, including the governments of Laos and South Vietnam in the 1970s.
Some are worried as well that after fighting the Taliban in Afghanistan for 18 years, US negotiations with the Islamist insurgents could lead to a deal that leaves the government in Kabul vulnerable to them.
Republican Senator Martha McSally, a former Air Force pilot who deployed to the Middle East six times, said that Trump's decision to give way to Turkey "is just wrong."
"The SDF, our Kurd allies, were the ones who paid a heavy price to fight that fight," she told Fox Radio.
"They're the ones that defeated the caliphate."
Not all in the US military community were opposed to Trump's move. Many support his desire to exit conflicts in the region after nearly two decades.
Dan Caldwell, a senior advisor at Concerned Veterans For America, said Trump was putting US interests first.
"It is not in our national interest to be in the middle of a long-running conflict between Turkey and Syrian Kurds that predates the rise of ISIS and the Syrian Civil War," he told AFP.
But Gallego stressed that Washington cannot assume other allies will trust the United States in the future.
"Outside of Israel, the strongest ally, and most consistent ally that we have had in the Middle East have been the Kurdish people... and we have just lost that," said Gallego.
"Abandoning the Kurds is another stark reminder that 'America First' means 'America Alone,'" he tweeted.