Mattis and Trump
Mattis and TrumpReuters

US President Donald Trump announced on Thursday that Defense Secretary Jim Mattis will be stepping down 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 equipment,” tweeted Trump.

“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 added.

The exact reason for Mattis’ departure is not yet known, but media outlets in the US published a copy of his resignation letter in which he said, "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.”

Some reports in the US hinted that Mattis’ departure could be related to Trump's announcement on the withdrawal of US forces from Syria. According to reports in the Washington Post, Mattis opposed the move and tried to persuade Trump not to go through with it.

Trump hinted in an interview in October that Mattis "could be" leaving and referred to him as "sort of a Democrat".

There was no knowledge of friction between the two, though veteran journalist Bob Woodward, in his recently published book “Fear”, claimed that Mattis told colleagues “the president acted like — and had the understanding of — a ‘fifth or sixth grader’” in relation to the nuclear standoff with North Korea.

Mattis called the quotes attributed to him “fiction” and the “product of someone’s rich imagination”.

Mattis’ resignation is the latest in a series of changes in Trump’s administration. Trump announced recently that White House Chief of Staff John Kelly will leave his post by the end of the year.

Initial reports indicated he was interested in appointing Nick Ayers, an aide to US Vice President Mike Pence, for the position, but Ayers subsequently announced that he would be leaving the White House at the end of year.

Mick Mulvaney was named Acting Chief of Staff by Trump last Friday.

Also stepping down is US Ambassador to the UN, Nikki Haley. Trump has nominated State Department spokeswoman Heather Nauert as her replacement.