In his two years as defense secretary, Mattis had disagreements with the president at moments, but it seemed to be Trump’s decision to pull US troops from Syria (paywall) that finally prompted Mattis to step down. Mattis’s letter offered a subtle but pointed rebuke of the president’s foreign policy, stating for example:

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.

The Times reports that, according to one aide, Trump had read the letter before he praised Mattis on Twitter, but he “did not understand just how forceful a rejection of his strategy Mr. Mattis had issued.” Over the past few days he had become more irate, until deciding that he would replace Mattis well before the former four-star general had originally planned to leave.

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