WASHINGTON — Former Defense Secretary Jim Mattis warns in a book excerpt of the bitter political divisions that seem to be tearing apart American society, echoing themes he often cited before he resigned from the Trump administration in protest.

The retired Marine general, who quit in December amid policy disagreements with President Donald Trump, says he is concerned about the state of American politics.

“We all know that we’re better than our current politics,” Mattis wrote in the excerpt published Wednesday in The Wall Street Journal. “Tribalism must not be allowed to destroy our experiment.”

Mattis said that “our own commons seems to be breaking apart” to a degree not seen in the past.

“What concerns me most as a military man is not our external adversaries; it is our internal divisiveness,” he wrote.

Much of the excerpt is a recitation of the reasons Mattis has previously given for agreeing to become the Pentagon chief despite not having known or spoken to Trump before being interviewed for the position in November.

Regarding his reasons for leaving, Mattis offered a slightly more pointed explanation than what his resignation letter outlined.

“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.

Mattis resigned shortly after Trump announced he was pulling all U.S. troops from Syria. In Mattis’ view this amounted to betraying the Syrian Kurdish fighters who’d partnered with American troops to combat the Islamic State group.

Mattis’ book, “Call Sign Chaos: Learning to Lead,” is scheduled to be published Sept. 3.