(RNN) - President Donald Trump tweeted Thursday that Secretary of Defense James Mattis will be retiring at the end of February.
“General Mattis was a great help to me in getting allies and other countries to pay their share of military obligations,” the president tweeted, “A new Secretary of Defense will be named shortly. I greatly thank Jim for his service!”
But according to the letter the secretary submitted to the president, it is, in fact, a resignation, effective at the end of February. Mattis said he was stepping down over differences with the president.
“Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position,” the letter said.
Trump announced Wednesday the U.S. would withdraw from Syria, declaring ISIS defeated. Some military and diplomatic leaders, however, have reportedly protested that decision.
The letter also stressed the importance of U.S. cooperation with allies, and cited both China and Russia as adversaries that “want to shape a world consistent with their authoritarian model.” Trump has been critical of American allies “not paying their fair share” when it comes to defense expenditures.
The letter specified Feb. 28 as his final day.
Mattis became secretary of defense shortly after the inauguration of Trump and is one of the longest-serving cabinet members.
“I am proud of the progress that has been made over the past two years on some of the key goals articulated in our National Defense Strategy: putting the Department on a more sound budgetary footing, improving readiness and lethality in our forces, and reforming the Department’s business practices for greater performance,” Mattis said in his letter.
Sen. Ben Sasse, R-NE, called it a “sad day.”
“General Mattis was giving advice POTUS needs to hear,” Sasse tweeted. “Mattis rightly believes that Russia & China are adversaries, and that we are at war with jihadists across the globe who plot to kill Americans. Isolationism is a weak strategy that will harm Americans.”
Before joining the Trump administration, Mattis served 44 years in the Marine Corps and earned his nickname, “Mad Dog,” for a brash and hard-charging style as a military commander.
“Be polite, be professional, but have a plan to kill everybody you meet," he famously said.
He had, however, reportedly begun to frustrate Trump with a more cautious approach as defense secretary.
Like many in the administration, he had been the subject of talk that his time as a member of the cabinet was growing short amid an expected shake-up by the president following the November midterms.
Seen as a moderating voice in the administration, tension between Mattis and Trump was detailed in Bob Woodward’s book, “Fear: Trump in the White House.”
In one of the instances, after explaining to Trump that the U.S. has forces in South Korea to prevent World War III, Mattis told close associates that “the president acted like - and had the understanding of - ‘a fifth- or sixth-grader,’” the Washington Post reported.
He also reportedly pushed back against a Trump request to assassinate Syrian President Bashar al-Assad.
Mattis came to the Trump White House after a much-heralded military career which included leading U.S. and British forces during the bloody Battle of Fallujah in Iraq in 2004.
He had also served under President Barack Obama from 2010-2013 as head of the critical U.S. Central Command, which oversees operations in the Middle East and Central Asia.
In that command, he disagreed with the Obama administration over Iran, saying the United States needed to prepare for conflict with the nation. He had also been an outspoken critic of the Iran nuclear deal.
His hawkish position on Iran was one of the foreign policy issues on which he and Trump were most closely aligned.
Mattis has also served as senior military assistant to the deputy Secretary of Defense; as director for Marine Corps Manpower Plans & Policy; as commanding general for Marine Corps Combat Development Command; and as executive secretary to the Secretary of Defense.
Mattis led a task force into Afghanistan in 2001, and was a commander of a NATO strategic command in Norfolk, VA.
After his retirement in 2013, he became a visiting fellow at the Hoover Institution at Stanford University, specializing in leadership, national security, strategy, innovation and the effective use of military force.