Just hours before President Volodymyr Zelensky of Ukraine sat down with President Trump in the Oval Office on Friday, Senator Lindsey Graham of South Carolina dispensed some advice to the Ukrainian leader.

“Don’t take the bait,” he said, encouraging him not to get into a spat with Mr. Trump.

“I said, don’t get into arguments about security agreements,” Mr. Graham recalled on Friday evening in a brief telephone interview with The New York Times, as he sat aboard Air Force One preparing to fly to Florida with the president.

Mr. Zelensky did not silence his concerns during his meeting with Mr. Trump, who has come to expect a level of capitulation from almost everyone who has met with him since Election Day, from foreign leaders to billionaire business executives. The result was an extraordinary dressing down by a U.S. president of a foreign ally in the middle of the Oval Office, while the media’s cameras recorded it all.

The question hovering over Washington on Friday evening was whether the confrontation was a spontaneous outburst or a planned verbal smack down by Mr. Trump and Vice President JD Vance, neither of whom respect Mr. Zelensky.

But three people with knowledge of what took place beforehand said neither Mr. Trump nor Mr. Vance had been looking to blow up a deal for Ukraine’s mineral rights, which Mr. Zelensky had been expected to sign in Washington. Instead, they said, Mr. Zelensky seemingly triggered the two American leaders by not sufficiently thanking the United States for trying to end the war (which Mr. Trump wanted to hear) and by pressing for commitments to protect Ukraine from Russian aggression going forward (which Mr. Trump did not want to hear).

In the end, Mr. Zelensky left the White House without a signed deal over mineral rights, which Mr. Trump had sought for weeks, and, for now, an even more contentious relationship with his country’s most important ally.