After the Hindenburg-like disaster that transpired in Brussels, and the diplomatic row that erupted in England, expectations for transatlantic unity were understandably dismal ahead of Donald Trump’s summit with Vladimir Putin in Helsinki. Trump had, after all, spent the preceding days antagonizing America’s allies, particularly at the annual convocation of NATO, prompting one diplomat in attendance to quip that the relationship had become akin to “battered-spouse syndrome.” Back in Washington, officials braced for the worst. Anything short of a “complete crisis,” one Senate aide told me, might be considered a win.

Instead, Trump somehow exceeded diplomats’ worst-case scenario for the summit, refusing to condemn Putin for meddling in the 2016 election and suggesting that his own intelligence agencies had erred in pointing the finger at Moscow. “They said they think it’s Russia. I have President Putin—he just said it’s not Russia,” Trump said. “I will say this: I don’t see any reason why it would be.” President Putin, Trump continued, “was extremely strong and powerful in his denial today.”

Onlookers across the political spectrum were stunned. “The enduring image of the Helsinki press conference will be Trump’s weakness standing next to Vladimir Putin. It was nothing short of cowardly for Trump to effectively side with Putin against our intelligence and law-enforcement communities . . . In my view, [it was a] dereliction of duty,” Nicholas Burns, the former U.S. ambassador to NATO, told me. “It may well be the most damaging trip by an American president in memory.”

Trump laid the groundwork for this capitulation early, tweeting that America’s relationship with Russia had “NEVER been worse” thanks to “years of U.S. foolishness and stupidity” and special counsel Robert Mueller’s investigation into his campaign. Even so, the spectacle that followed was flabbergasting. In the press conference after Trump and Putin’s private meeting, Trump launched into a conspiratorial rant in which he refused to denounce Russia’s election meddling, and appeared to blame Hillary Clinton and the F.B.I. for hiding evidence related to the hacking of the Democratic National Committee. “What happened to the servers of the Pakistani gentleman that worked on the D.N.C.? Where are those servers? They’re missing—where are they? What happened to Hillary Clinton’s e-mails? Thirty-three thousand e-mails gone—just gone,” he sputtered, at one point, rambling.

In fact, Trump added, changing topics, Putin had extended him an “incredible offer”: to allow Mueller’s agents to fly to Russia to work with Russian investigators and get to the bottom of whatever really happened. “I think that’s an incredible offer,” he repeated. “O.K.?”

Among current and former U.S. officials I spoke with, the uniform response was horror and dismay. “To see a U.S. president stand up and discredit a briefing on election meddling from his own director of national intelligence, and instead take the word of the Russian president, is absolutely shocking,” said Julianne Smith, who served as Joe Biden’s deputy national-security adviser. “I don’t have any other word other than ‘breathtaking.’ It took my breath away.” For decades, Smith added, “We had this old adage . . . that politics ends at the water’s edge”—that Americans will choose patriotism over party when the nation is threatened. “It’s clear that at this moment in history, that is now gone,” she sighed. “To have a president stand up and criticize Democrats, while on foreign soil meeting with the president of Russia, is just something that none of us are used to seeing—and never imagined we would see.”

“It’s the world turned upside down,” agreed Antony Blinken, who served as President Barack Obama’s deputy national-security adviser and deputy secretary of state. “Faced with a damning, detailed indictment of Russian meddling in the 2016 election, and a ‘blinking red’ warning from his own director of national intelligence that Russian attacks on our democracy continue to this day, instead of confronting Mr. Putin, President Trump puts his word on par with the intelligence community, attacks the investigators, picks fights with our allies, and blames America for bad relations with Russia.”