Former Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE warned against shunning allies and America breaking down into “tribalism” in his first extensive remarks since leaving office.

In an essay published Wednesday in The Wall Street Journal adapted from his forthcoming book “Call Sign Chaos: Learning to Lead,” Mattis lamented divisive leaders who fail to recognize the importance of allies.

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“Nations with allies thrive, and those without them wither,” Mattis wrote. “Alone, America cannot protect our people and our economy. At this time, we can see storm clouds gathering. 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 did not mention President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE by name in those sections of the essay. But when Mattis resigned as Defense secretary in December, his resignation letter to Trump stressed his differences with the president on the importance of allies.

In the Journal piece, Mattis also defended his tenure as secretary, saying, “I did as well as I could for as long as I could.”

“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,” he wrote.

Trump has often disparaged U.S. allies in NATO and elsewhere and has insisted they pay more for their own defense. Most recently, he sparked a feud with NATO ally Denmark over purchasing Greenland and brought turmoil to this weekend’s Group of Seven summit.

Mattis resigned after Trump announced a since-reversed plan to withdraw all U.S. troops from Syria, a move Mattis and others feared would leave U.S. partners such as the Kurds open to attack.

“An oft-spoken admonition in the Marines is this: When you’re going to a gunfight, bring all your friends with guns,” the retired Marine general wrote in the essay published Wednesday. “From imaginative military solutions to their country’s vote in the U.N., the more allies the better. I have never been on a crowded battlefield, and there is always room for those who want to be there alongside us.”

Mattis also warned about divisions among Americans, saying what concerns him most “as a military man” is not adversaries but “our internal divisiveness.”

“We are dividing into hostile tribes cheering against each other, fueled by emotion and a mutual disdain that jeopardizes our future, instead of rediscovering our common ground and finding solutions,” he wrote.

“All Americans need to recognize that our democracy is an experiment—and one that can be reversed,” he continued. “We all know that we’re better than our current politics. Tribalism must not be allowed to destroy our experiment.”