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 respects Mr. Zelensky.