Former Defense Secretary James Mattis deplored today’s political climate, saying it poses more of a threat to our democracy than our adversaries abroad.

In an essay published Wednesday in the Wall Street Journal, Mattis warned Americans that democracy is an experiment “that can be reversed” if the divisiveness continues.

“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.

“We all know that we’re better than our current politics. Tribalism must not be allowed to destroy our experiment,” he said.

For it to survive, Mattis said, Americans must live by the motto “E Pluribus Unum” — from many, one.

Mattis, 68, has mostly kept quiet since he resigned in December in protest of President Trump’s decision to withdraw American forces from Syria and the president’s treatment of U.S. allies.

The longtime Marine also criticized Trump’s embrace of isolationism that has been a centerpiece of his presidency.

"Nations with allies thrive, and those without them wither. 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. Returning to a strategic stance that includes the interests of as many nations as we can make common cause with, we can better deal with this imperfect world we occupy together. Absent this, we will occupy an increasingly lonely position, one that puts us at increasing risk in the world,” Mattis wrote.

Mattis’ book, Call Sign Chaos: Learning to Lead, is out Tuesday.