Donald Trump was dangerously isolated on Monday as, in a rare rebuke, some of his most loyal allies revolted against his decision to withdraw US troops from north-eastern Syria.

Senate majority leader Mitch McConnell led a chorus of Republicans who, having defended the president on almost every other issue – including over impeachment – decided to draw a line in the sand.

“A precipitous withdrawal of US forces from Syria would only benefit Russia, Iran, and the Assad regime,” McConnell said. “And it would increase the risk that Isis and other terrorist groups regroup.”

He added: “As we learned the hard way during the Obama administration, American interests are best served by American leadership, not by retreat or withdrawal.”

The criticism was significant because McConnell is usually at pains not to cross Trump even at his most capricious. Last week the Kentucky senator released a Facebook video promising to stop Democratic-led impeachment in its tracks.

Q&A How do you impeach the US president? Show Article 1 of the United States constitution gives the House of Representatives the sole power to initiate impeachment and the Senate the sole power to try impeachments of the president. A president can be impeached if they are judged to have committed "treason, bribery, or other high crimes and misdemeanors" – although the US Constitution does not specify what “high crimes and misdemeanors” are.

The formal process starts with the House of Representatives passing articles of impeachment, the equivalent of congressional charges. According to arcane Senate rules, after the House notifies the Senate that impeachment managers have been selected, the secretary of the Senate, Julie Adams, tells the House that the Senate is ready to receive the articles. Then impeachment managers appear before the Senate to “exhibit” the articles, and the Senate confirms it will consider the case. The presiding officer of the Senate notifies the supreme court chief justice, John Roberts, of the impending trial. Roberts arrives in the Senate to administer an oath to members. The presiding officer will then administer this oath to senators: “I solemnly swear that in all things appertaining to the trial of the impeachment of Donald Trump, now pending, I will do impartial justice according to the constitution and laws, so help me God.” The Senate must vote on a resolution laying out ground rules for the trial including who the key players will be, how long they will get to present their cases and other matters. After the Senate is “organized”, the rules decree, “a writ of summons shall issue to the person impeached, reciting said articles, and notifying him to appear before the Senate upon a day and at a place to be fixed by the Senate”. A president has never appeared at his own impeachment trial. Trump will be represented by the White House counsel, Pat Cipollone, and his personal lawyer Jay Sekulow, among others. After the oath, the trial proper will begin. Senators may not speak during the proceedings but may submit written questions. The question of witnesses and other matters would be decided on the fly by majority vote. A time limit for the proceedings will be established in the initial Senate vote. The senators will then deliberate on the case. In the past this has happened behind closed doors and out of public view. The senators vote separately on the two articles of impeachment – the first charging Trump with abuse of power, the second charging him with obstruction of Congress. A two-thirds majority of present senators – 67 ayes if everyone votes – on either article would be enough to convict Trump and remove him from office. But that would require about 20 Republicans defections and is unlikely. The more likely outcome is a Trump acquittal, at which point the process is concluded. Two presidents have previously been impeached, Bill Clinton in 1998, and Andrew Johnson in 1868, though neither was removed from office as a result. Richard Nixon resigned in 1974 before there was a formal vote to impeach him. Tom McCarthy in New York

The unusual fracture emerged on Sunday night when, shortly after a phone conversation between Trump and Turkey’s president Recep Tayyip Erdoğan, the White House announced removal of US troops from the Syria-Turkey border area. “Turkey will soon be moving forward with its long-planned operation into Northern Syria,” it added.

Critics of all political stripes have long feared that the move could open the way for a Turkish strike on Kurdish-led fighters in the area. Kurdish groups have fought alongside a small US presence in Syria to drive Islamic State militants from the region.

The Republican backlash was rapid and potentially unnerving for a president whose fate is tethered to the party and the assumption that it will acquit him in the Senate if, as widely expected, the Democratic-led House of Representatives votes for impeachment.

Lindsey Graham, the chairman of the Senate judiciary committee, who has become an outspoken defender (and frequent golf partner) of Trump, did not acquiesce this time. Abandonment of the Kurds would be “a disaster in the making”, he said, and “a stain on America’s honour”.

Graham told Fox News: “I hope I’m making myself clear how short-sighted and irresponsible this decision is. I like President Trump. I’ve tried to help him. This, to me, is just unnerving to its core.”

Graham wrote on Twitter that if the plan goes ahead, he will introduce a Senate resolution opposing it and seeking reversal of the decision. He added: “We will introduce bipartisan sanctions against Turkey if they invade Syria and will call for their suspension from NATO if they attack Kurdish forces who assisted the US in the destruction of the ISIS Caliphate.”

Kevin McCarthy, the top Republican in the House, whose attempts to defend Trump’s phone call with Ukraine’s president have provoked mockery, said: “If you make a commitment and somebody is fighting with you, America should keep their word.”

Michael McCaul of Texas, the lead Republican on the House foreign affairs committee, also urged the president to reconsider. “The United States should not step aside and allow a Turkish military operation in north-east Syria,” he said. “This move will undermine our ongoing campaign to prevent an Isis resurgence and will ultimately threaten our homeland.

“Additionally, the United States needs to stay engaged to prevent further destructive involvement in the region from our adversaries like the Assad regime, Putin and Iran.”

Notably, senator Marco Rubio of Florida, reluctant to criticise Trump even when the president suggested that China investigate former vice-president and 2020 election rival Joe Biden, was clear , describing the retreat as “a grave mistake that will have implications far beyond Syria”.

And Nikki Haley, Trump’s former UN ambassador, admonished Trump without mentioning his name. “We must always have the backs of our allies, if we expect them to have our back,” she tweeted. “The Kurds were instrumental in our successful fight against Isis in Syria. Leaving them to die is a big mistake. #TurkeyIsNotOurFriend”

Ominously for Trump, even conservative Fox News aired dissent. Host Brian Kilmeade described the pullout as “a disaster”, telling viewers of Fox & Friends: “Abandon our allies? That’s a campaign promise? Abandon the people that got the caliphate destroyed?”

Republicans who have contradicted Trump before did so forcefully again. Utah senator Mitt Romney described Trump’s announcement as “a betrayal”, adding: “It says that America is an unreliable ally; it facilitates ISIS resurgence; and it presages another humanitarian disaster.”

Romney and Democratic senator Chris Murphy issued a joint statement insisting Trump’s administration “explain to the American people how betraying an ally and ceding influence to terrorists and adversaries is not disastrous for our national security interests”.

Democrats also piled in but there was a lone voice of support for the president on Capitol Hill. Republican senator Rand Paul, long a critic of foreign intervention, said: “So many neocons want us to stay in wars all over the Middle East forever. [Trump] is absolutely right to end those wars and bring the troops home.”

Trump himself was undeterred by the blowback. Speaking at the White House on Monday, he said he has “great respect” for the prominent Republican critics.

And added: “People are extremely thrilled because they say it’s time to bring our people back home. We’re not a police force. They’re policing the area. We’re not a police force. The UK was very thrilled at this decision … many people agree with it very strongly.”