For decades, American historians have viewed the summit between President John F. Kennedy and Soviet Premier Nikita Khrushchev, in 1961, as the worst ever between Washington and Moscow. Kennedy described the encounter, in Vienna, as one of the low points of his life; his weakness led Khrushchev to test the United States by deploying nuclear-armed missiles in Cuba. That disastrous bit of summitry has now been topped by President Trump’s meeting on Monday with the Russian leader, Vladimir Putin, and his public renunciation of U.S. intelligence reports and a recent Justice Department indictment of Russian officials for meddling in the 2016 election. Senator John McCain called Trump’s remarks “one of the most disgraceful performances in memory” by a U.S. President. John Brennan, the former C.I.A. director, said Trump’s deference to Putin was “nothing short of treasonous.” In a tweet, he charged that Trump’s comments were “imbecilic” and that they indicate the President is “wholly in the pocket of Putin.” His action “rises to & exceeds the threshold of ‘high crimes & misdemeanors.’ ” Brennan added, “Republican Patriots: Where are you???”

Despite the Justice Department’s indictment, on Friday, of twelve Russian intelligence agents, for interfering in the 2016 election, Trump fully embraced Putin’s bare-faced denial. “He just said it’s not Russia,” Trump said at a joint press conference, after the meeting. “I will say this: I don’t see any reason why it would be.” He even heralded—as an “incredible offer”—Putin’s suggestion that the special counsel, Robert Mueller, come to Moscow to question the twelve agents, with Russian officials present, in exchange for Russia being allowed to interrogate any U.S. intelligence officials it believes have been involved in covert operations against Russia.

In Helsinki, Trump may have done far more damage than Kennedy did, Robert Kagan, a former State Department policy-planning staffer who is now at the Brookings Institution, told me. “Whereas Kennedy in the end was trying to strengthen the American position, Trump is actively and deliberately weakening it,” Kagan said. “By undermining our alliances and destroying the American-led world order, he is leading us back toward the kind of dangers that we saw in the first half of the twentieth century. There may not be anything so dramatic as the Cuban missile crisis right away—Russia is not in the position the Soviet Union was in—but over time the costs and dangers are likely to be much higher.”

Trump has changed the policy of every American President, Republican or Democrat, since the United States and Russia became global rivals and nuclear powers, Kagan said. “We have never before had an American President who shared Moscow’s goals,” he added.

Richard Burt, the chief negotiator of the 1991 Strategic Arms Reduction Treaty with the Soviet Union and a longtime adviser to Republican candidates, called Trump’s diplomacy “more P. T. Barnum than Henry Kissinger.” The President’s victories are “all hype,” he added.

“In the longer run, he creates some potentially very serious problems. With our friends, he is testing the understanding and patience of our closest allies. With our adversaries, he looks like all bark and no bite,” Burt said. Trump has “adapted serious diplomacy to superficial reality TV.”

The Helsinki summit follows two months of rocky performances by the President on the international stage. At the G-7 summit of the world’s most powerful economies, Trump flew out early, reneged on signing a joint communiqué outlining common goals, and insulted the host, the Canadian Prime Minister, Justin Trudeau, on Twitter. At the historic summit with the North Korean dictator, Kim Jong Un, Trump engaged in great theatre but got only a vague promise, with no specifics, that Pyongyang will denuclearize. At the NATO summit last week, Trump insulted the German Chancellor, Angela Merkel, as a “captive of the Russians” and demanded, unrealistically, that the twenty-eight other nations double, or more, their contributions to the world’s largest military alliance, then left the room. In Britain, he embarrassed Prime Minister Theresa May by telling a British tabloid how she should conduct Brexit negotiations, clumsily violated protocol with Queen Elizabeth, and generated headlines by sitting smugly for a photograph in Winston Churchill’s old chair. On Sunday, Trump called the European Union “a foe.” Now Helsinki.

“The last three months have substantially weakened the U.S. position in the world,” Richard Haass, the president of the Council on Foreign Relations, told me. “We are in a trade war with our most important economic partners, have created doubts in the minds of the European allies (as a result of our harangues over defense spending and our withdrawal from the Iran nuclear deal, the J.C.P.O.A.) as to U.S. reliability and our willingness to speak truth to Russian power, and have failed to move North Korea closer to denuclearization while weakening sanctions and raising doubts in Seoul as to U.S. dependability.”

Putin clearly tried to play alpha dog with Trump from the start. At an opening photo call, the optics were awful. Putin looked bored. He slouched back into the side of his chair, his legs spread. He often didn’t even look at Trump. Trump, wearing his omnipresent red tie, sat forward in his chair. His fingers tapped against each other, perhaps instinctively, perhaps nervously. Notably missing in comments at either of their public appearances was any mention of Russia’s alleged use of chemical weapons in Britain, its alleged shooting down of Malaysia Airlines Flight 17—the fourth anniversary is Tuesday—over Ukraine, which killed nearly three hundred people, or its alleged interference in European elections.

Trump accomplished little in the way of substance in Helsinki. “Putin wanted to communicate that a new era in U.S.-Russian relations had begun, something Trump was all too happy to associate himself with,” Haass told me. “Trump engaged in moral equivalence in blaming both countries for the poor state of relations. As was the case in Singapore, he exaggerated what had been accomplished by the summit. Most egregiously, he criticized the Mueller investigation and refused to back his own intelligence and law-enforcement agencies.” The one positive development may be movement on outstanding arms-control issues, although neither leader provided any details.

Nina Khrushcheva, the granddaughter of the former Soviet premier, called the Trump-Putin summit a “photo op for both, a victory story that serves their respective narratives.” Now an American citizen and professor of international relations, at the New School, in New York, Khrushcheva worked as a research assistant to the legendary American diplomat George Kennan. “As Kennan used to say,” she told me, “all foreign-policy decisions should never serve interests of the moment and look forward to at least five, ten years. With Trump, this is all for the moment. For Putin, for as long as he needs to stay in power.” She added, “I am seeing Trump as a follower rather than a leader in this relationship.”