Mattis resigned amid reports that he is unhappy with the president’s push for a quick withdrawal from Syria. Commenting on the matter, President Donald Trump praised Mattis’s service:
....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!
— Donald J. Trump (@realDonaldTrump) December 20, 2018
There are speculations that Mattis’s resignation wasn’t voluntary and that he was fired:
@barbarastarrcnn, Pentagon correspondent for CNN, Jim Mattis is not retiring, he is being "pushed out."
— Namo Abdulla (@namo_abdulla) December 20, 2018
Mattis’s resignation letter is notable for scanting any praise of the president (as is customary in the genre) and highlighting policy differences.
Part of the letter reads:
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. Like you, I have said from the beginning that the armed forces of the United States should not be the policeman of the world. Instead, we must use all tools of American power to provide for the common defense, including providing effective leadership to our alliances. 29 democracies demonstrated that strength in their commitment to fighting alongside us following the 9-11 attack on America. The Defeat-ISIS coalition of 74 nations is further proof.
Similarly, I believe we must be resolute and unambiguous in our approach to those countries whose strategic interests are increasingly in tension with ours. It is clear that China and Russia, for example, want to shape a world consistent with their authoritarian model gaining veto authority over other nations economic, diplomatic, and security decisions to promote their own interests at the expense of their neighbors, America and our allies. That is why we must use all the tools of American power to provide for the common defense.
My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues. We must do everything possible to advance an international order that is most conducive to our security, prosperity and values, and we are strengthened in this effort by the solidarity of our alliances.
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
It’s hard to read this passage as anything other than a repudiation of Trump’s foreign policy.