Defense Secretary Jim Mattis is stepping down from his post after clashing with the president over a troop drawdown in Syria and Trump's go-it-alone stance in world affairs -- news that prompted a range of reactions from lawmakers.
Mattis, perhaps the most respected foreign policy official in Trump's administration, will leave by the end of February after two tumultuous years struggling to soften and moderate the president's hardline and sometimes abruptly changing policies.
In his resignation letter, the retired Marine general alluded to disagreements with Trump as the reason for his departure.
Among prominent Republicans, Senate Majority Leader Mitch McConnell said he was sorry to learn of Mattis' departure and urged the president to "select a leader who shares Secretary Mattis' understanding" of vital American foreign policy principles.
McConnell also said he was distressed and expressed concern that without Mattis' leadership, the country's carefully cultivated alliances around the world will not sustain.
"I believe it’s essential that the United States maintain and strengthen the post-World War II alliances that have been carefully built by leaders in both parties," McConnell's statement said. "We must also maintain a clear-eyed understanding of our friends and foes, and recognize that nations like Russia are among the latter.
"So I was sorry to learn that Secretary Mattis, who shares those clear principles, will soon depart the administration," McConnell continued. "But I am particularly distressed that he is resigning due to sharp differences with the president on these and other key aspects of America’s global leadership.
"It is regrettable that the president must now choose a new Secretary of Defense. But I urge him to select a leader who shares Secretary Mattis’s understanding of these vital principles and his total commitment to America’s servicemembers."
Sen. Lindsey Graham of South Carolina tweeted that Mattis "has been in the fight against radical Islam for decades and provided sound and ethical military advice to President Trump."
Florida Sen. Marco Rubio tweeted, "Just read Gen. Mattis resignation letter. It makes it abundantly clear that we are headed toward a series of grave policy errors which will endanger our nation, damage our alliances & empower our adversaries."
House Speaker Paul Ryan of Wisconsin said of Mattis in a statement: "With our country’s defense in his hands, we have become a safer America at home, and a more commanding force abroad. His patriotism and grit are matched only by his humility and graciousness."
"Defense Secretary Mattis has served our country with honor and distinction for many years," tweeted Republican Sen. Susan Collins of Maine. "His departure is a real loss for our nation, our troops, and this Administration."
Democratic Rep. Nancy Pelosi of California said she was "shaken" by the news.
"This is very serious for our country," Pelosi said.
Democratic Sen. Chris Murphy of Connecticut referred to the resignation as a "national security crisis."
"A Secretary of Defense quitting over a public disagreement with a President whose foreign policy he believes has gone off the rails is a national security crisis," he tweeted. "No way around it."