"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 as reported by CNN.

A senior administration official told CNN that Mattis was "vehemently opposed" to the Syria decision and a possible Afghanistan troop withdrawal. 

Meanwhile, Trump tweeted that Mattis will retire at the end of February.

"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. During Jim’s tenure, tremendous progress has been made, especially with respect to the purchase of new fighting," Trump tweeted.

The President did not name his successor.

"Equipment. General Mattis was a great help to me in getting allies and other countries to pay their share of military obligations. A new Secretary of Defense will be named shortly. I greatly thank Jim for his service!," he said.