Defense Secretary Jim Mattis will step down at the end of February, according to a resignation letter he hand-delivered to President Donald Trump Thursday, a day after Trump overruled his advice against pulling troops out of Syria.
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 sharply changing policies.
In the letter, Mattis wrote of differences with the president on maintaining alliances, advancing an international order and using American power for a common defense. China and Russia want to shape a world consistent with their authoritarian model at the expense of the United States and its allies, he wrote.
"We must do everything possible to advance an international order that is most conducive to our security, prosperity and values, and we are strengthened in this effort by the solidarity of our alliances," Mattis wrote.
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Trump has the right to a secretary of defense whose views are better aligned with his on "these and other subjects," he wrote.
His departure was immediately lamented by foreign policy hands and lawmakers on both sides of the aisle, who viewed the retired Marine general as a sober voice of experience in the ear of a president who had never held political office or served in the military. Even Trump allies expressed fear over Mattis' decision to quit, believing him to be an important moderating force on the president.
"Just read Gen. Mattis resignation letter," Republican Florida Sen. Marco Rubio tweeted. "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."
Democratic Rep. Nancy Pelosi of California said that she was "shaken" by the news.
"I am shaken by the news because of the patriot that General Mattis is," she said. "I think that everybody in the country should read his letter of resignation. It’s a letter of great patriotism, great respect for the president but also a statement of his values."
The president tweeted in announcing Mattis' departure, which he characterized as a retirement, that a new defense secretary "will be named shortly."
"During Jim's tenure, tremendous progress has been made, especially with respect to the purchase of new fighting equipment," he wrote. "General Mattis was a great help to me in getting allies and other countries to pay their share of military obligations."
Trump said Wednesday that the United States had beaten ISIS in Syria, but experts and former officials disagreed. They warned that Trump's surprise decision to withdraw troops from Syria would be seen as a victory for Russia and Iran and a betrayal of Kurdish and Arab forces fighting a seven-year war against the Damascus regime.
Mattis met with the president Thursday afternoon to tell him he would be resigning and deliver the letter, a Defense Department spokesman said. A senior White House official told NBC News Mattis and the president had differences of opinion over some issues over the last few months but did not know whether Syria had specifically come up.
Republican Sen. Lindsey Graham of South Carolina said earlier Thursday that Mattis had told him that this was not the time to leave Syria, and that "a replay of Iraq was very likely." Graham, who called the decision to remove troops from Syria an Obama-like mistake, had urged Trump to follow military advice in the fight against ISIS.
Of Mattis, he tweeted Thursday, "He has been in the fight against radical Islam for decades and provided sound and military ethical advice to President Trump."
Mattis and Trump had clashed over a number of international decisions. He defended the Iran nuclear deal just as Trump pulled out of it and was at odds with the president’s opposition to U.S. military exercises with South Korea in a concession to North Korea’s Kim Jong Un.
Mattis, 68, is a Marine Corps general who retired in 2013 after serving as the head of the U.S. Central Command.
He has a reputation as a battle-hardened, tough-talking Marine who was entrusted with some of the most challenging commands in the U.S. military.
Mattis was only the second retired general to serve as defense secretary, the first being George C. Marshall in 1950-51 during the Korean War. Marshall was a much different figure, having previously served as U.S. secretary of state and playing a key role in creating closer ties with Western Europe after World War II.
Although his record in combat and his credentials as a senior commander are widely admired, Mattis had little experience in the diplomatic aspects of the job of secretary of defense upon taking the position.
Born in Pullman, Washington, Mattis enlisted in the Marine Corps in 1969, later earning a history degree from Central Washington University. He was commissioned as an officer in 1972. As a lieutenant colonel, Mattis led an assault battalion into Kuwait during the first U.S. war with Iraq in 1991.
As head of the Central Command from 2010 until his retirement in 2013, he was in charge of both the wars in Iraq and Afghanistan.
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The Associated Press contributed to this article.