Friday's shocking scene in the Oval Office blew up a years-long wartime alliance between pro-Western Ukraine and the United States.
Zelensky, previously hailed in Washington as a Churchillian figure, was shouted at by Trump and Vice President JD Vance, then ejected from the White House without lunch.
And also without signing a US-Ukraine minerals sharing deal, seen as key to American backing for a truce with Russia.
Zelensky's sin?
Publicly contradicting Trump on facts of the Ukraine war, then refusing to back down.
"He disrespected the United States of America in its cherished Oval Office," Trump posted on social media.
Since returning for a second term, Trump has stretched the norms and even legal boundaries of how much power a US president can wield at home.
In foreign policy, he is also making clear that the days of consensus-building are over. Now, it's whatever he says goes.
Nowhere is that shift starker than on Ukraine.
Trump's U-turn -- in which Russia is more partner than pariah and Ukraine more a client than ally -- brought French President Emmanuel Macron and British PM Keir Starmer rushing to the White House this week.
Like Zelensky, they worry that Trump wants to force Kyiv into a peace deal that gives Russian President Vladimir Putin what he wants, while leaving Ukraine broken and insecure.
But they both knew they had to cater to Trump's demand to be treated as a leader who shouldn't be questioned.
Macron engaged Trump in a series of handshakes, knee touching and back slapping that clearly pleased his host.
This helped to soften the tension when the French president publicly corrected Trump after he repeated one of his most frequent falsehoods -- that Europe gives far less money than Washington to Ukraine and will get it all back.
Days later, Starmer heard the same disinformation and also corrected Trump. Yet Starmer already had delighted Trump with a signed invitation from King Charles III to visit Britain.
In Zelensky's case, the stakes were entirely different.
Rather than correcting Trump on the relatively minor issue of who's paying what, he tried to push back on the White House's entire embrace of an increasingly pro-Moscow narrative.
Even before he arrived, Trump and his Republican allies had taken to calling Zelensky a dictator and echoing false Kremlin claims that Russia didn't start the war.
Then, in front of journalists invited for the occasion, Trump insisted to Zelensky that he would have to compromise with Putin and chided him for being hostile to the man Zelensky called a "killer."
When Vance chimed in, calling Zelensky ungrateful and an obstacle to diplomacy, the Ukrainian made his fateful decision to debate.
"What kind of diplomacy, JD?" he asked Vance, enumerating diplomatic initiatives over the years that have failed to stop Moscow's military encroachments.
Vance angrily called Zelensky "disrespectful" and before long he and Trump were tag-teaming against their would-be ally.
- Would flattery have worked? -
Trump's followers immediately blamed Zelensky.
"What kind of dictator would insult President Trump and VP Vance while begging for money to fund a war he could never win?" wrote @GuntherEagleman, a far-right Trump supporter whose real name is David Freeman and has 1.3 million followers on X.
Marjorie Taylor Greene, a hardcore Trump supporter in Congress, called Zelensky "arrogant."
Her boyfriend Brian Glenn, who was in the Oval Office as a reporter for right-wing Real America's Voice television, mockingly asked Ukraine's president why he wasn't wearing a suit.
"I will wear (a suit) after this war finishes," Zelensky, who wears military-style outfits, answered wearily.
Some analysts said Zelensky should have been more strategic.
"The only thing Zelensky should have said in public -- no matter what the question -- was 'Thank you, Mr President. Thank you, America," Fox News commentator and retired general Jack Keane said. "Over and over."
CNN foreign policy expert Fareed Zakaria said Zelensky's first words should have been that Trump was "a genius."
But for Democratic Senator Chris Murphy, no amount of diplomatic skills or flattery would have saved Zelensky, because Trump was pushing a settlement that "hands Ukraine to Putin" -- and Zelensky knew it.
The Oval Office was an "ambush," Murphy wrote on X.
