"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," Mattis wrote in his letter to the President

Earlier Thursday, a senior administration official told CNN's Jake Tapper that Mattis was "vehemently opposed" to the Syria decision and a possible Afghanistan troop withdrawal. Mattis did not explicitly cite his opposition to the President's planned withdrawal of US troops from Syria -- which caught US allies off guard -- but the retired four-star general was privately adamant in urging Trump against the pullback. Read more: Defense Secretary Jim Mattis quits, says his views aren't 'aligned' with Trump as president upends major US policies

On Thursday, President Donald Trump tweeted that Mattis was "retiring, with distinction, at the end of February." The President touted the "tremendous progress" that has been made during Mattis' tenure at the helm of the Defense Department and thanked him for his service. In a sign of the swift nature of the resignation, senior White House officials found out about it from the President's tweet, according to two officials. "It is regrettable that the president must now choose a new Secretary of Defense. On Thursday, an official said the president was planning to withdraw at least 5,000 of the 14,000 U.S. troops in Afghanistan - another blow to Mattis, who advocated for a strong U.S. military presence to bolster diplomatic peace efforts.
U.S. Secretary of Defence Jim Mattis announces his resignation at the end of February 2019. Mattis says that his views are not well "aligned" with those of the President.