Sen. Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-S.C.) says he confronted President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s acting Pentagon chief in a heated exchange over the administration’s Syria strategy.

Speaking to acting Defense Secretary Patrick Shanahan Patrick Michael ShanahanHouse Armed Services chairman expresses confidence in Esper amid aircraft carrier coronavirus crisis Boeing pleads for bailout under weight of coronavirus, 737 fallout Esper's chief of staff to depart at end of January MORE at a briefing at the Munich Security Conference last weekend, the senator said he cursed at Shanahan and declared him an adversary.

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Graham described his remarks to Washington Post columnist Josh Rogin, saying he told Shanahan that the Trump administration's plan to withdraw all U.S. troops from Syria by the end of April was “the dumbest f---ing idea I’ve ever heard.”

“Well, if the policy is going to be that we are leaving by April 30, I am now your adversary, not your friend,” Graham said he told Shanahan.

Officials told NBC News about the exchange, with some lawmakers saying the briefing left them with less confidence in Shanahan, and one describing the episode with Graham as “pretty tense.”

Shanahan reportedly further frustrated lawmakers by refusing to state his opinion on President Trump’s Syria decision.

A Defense Department official told NBC News that the meeting “ended on a positive note” despite the tense exchange.

Shanahan stepped into the role of Defense chief after James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE abruptly left the administration in what was seen as a rebuke of Trump’s Syria strategy.

Graham has been one of the harshest Capitol Hill critics of Trump’s Syria withdrawal plan. He continued that criticism after the exchange with Shanahan, telling the Post’s Rogin: “After Iraq, not only would this be a mistake, this would be almost an unpardonable sin, in the sense that you know what can happen. This can risk his whole presidency.”