(CNN) -- Defense Secretary James Mattis resigned Thursday on the heels of President Donald Trump's plans to withdraw troops from Syria, citing irreconcilable policy differences.
"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.
He also pointedly stated that the strength of the US depends in part on the strength of its alliances around the globe, many of which have become notably frayed under Trump.
"One core belief I have always held is that our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships. While the US remains the indispensable nation in the free world, we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies," he added.
Trump first announced Mattis' departure in a tweet.
"General Jim Mattis will be retiring, with distinction, at the end of February, after having served my Administration as Secretary of Defense for the past two years," Trump tweeted.
Trump touted the "tremendous progress" that has been made during Mattis' tenure at the helm of the Defense Department and thanked Mattis for his service.
Trump said a successor "will be named shortly."
Mattis and the President's other top national security advisers opposed Trump's decision to withdraw from Syria.