Former Defense Secretary Jim Mattis took several implicit swipes at President Trump’s leadership in an op-ed published Wednesday, warning about the fallout from fractured alliances and increasing divisiveness under the current administration.

Mattis’ comments were included in a lengthy essay adapted from a forthcoming book and published in The Wall Street Journal. They amount to the most extensive remarks from him about his time in the Trump administration since he quietly resigned in January.

In the essay, Mattis suggested he left his post as secretary of defense amid concerns about “keeping faith with our allies,” warning that America “cannot go it alone.”

“Nations with allies thrive, and those without them wither. Alone, America cannot protect our people and our economy,” Mattis wrote. “At this time, we can see storm clouds gathering.”

He pointedly added: “A polemicist’s role is not sufficient for a leader. A leader must display strategic acumen that incorporates respect for those nations that have stood with us when trouble loomed.”

Mattis said he “did as well as I could, for as long as I could” as secretary of defense.

In one section, he essentially wrote that he resigned when he felt his concerns about those alliances were not being taken seriously.

“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,” Mattis wrote.

Mattis went on to note his deepest concerns “as a military man,” noting they are “not our external adversaries; it is our internal divisiveness.”