Former Secretary of Defense James Mattis is speaking out in a new essay after leaving the Trump administration late last year in what President Trump described as "essentially" a firing.

Mattis, who announced his resignation in December 2018 after disagreeing with Trump's decision to withdraw troops from Syria, published the new essay, an excerpt of his forthcoming book, in The Wall Street Journal. In it, he reflects on the sense of duty he felt when being selected for the administration position, discusses his decision to resign, and offers a few warnings that read as criticisms of Trump.

"Using every skill I had learned during my decades as a Marine, I did as well as I could for as long as I could," Mattis writes of his time in the Trump administration. "When my concrete solutions and strategic advice, especially keeping faith with our allies, no longer resonated, it was time to resign, despite the limitless joy I felt serving alongside our troops in defense of our Constitution."

Although Mattis doesn't criticize Trump directly, he warns that nations without allies "wither," saying that a leader must show "respect" for these allies and warns of the United States occupying an "increasingly lonely position" that "puts us at increasing risk in the world." Mattis' essay was notably published the same week Trump reportedly clashed with allies at the Group of Seven summit by arguing for Russia's readmission, and a week after he canceled a diplomatic trip to Denmark over the prime minister rebuffing his idea to purchase Greenland.

Mattis also writes in a line that certainly sounds directed at Trump that "a polemicist's role is not sufficient for a leader."

Mattis offered similar recommendations about treating allies with respect in his resignation letter last year. Trump was reportedly so angry at the letter that he removed Mattis from his position two months early. Brendan Morrow