"An oft-spoken admonition in the Marines is this: When you're going to a gunfight, bring all your friends with guns," Mattis wrote in the Wall Street Journal, in his first public comments since quitting in a policy dispute with Trump last December.

"A leader must display strategic acumen that incorporates respect for those nations that have stood with us when trouble loomed," Mattis wrote.

"Nations with allies thrive, and those without them wither. Alone, America cannot protect our people and our economy."

Mattis's opinion column was published just hours before his successor, Mark Esper, gives his first on-camera press briefing since being confirmed as Pentagon chief last month.

Esper has the responsibility of implementing Trump's demands for a withdrawal of most US troops in Afghanistan, Iraq and Syria, the policies Mattis fell out with the president over.

Mattis, a retired Marine Corps general, said he had to step down after his "concrete solutions and strategic advice, especially keeping faith with our allies, no longer resonated," with the US leadership.

Mattis also blasted the hyper-partisan political atmosphere in Washington as contributing to the erosion of US leadership in the world, saying he had little taste for "the political fratricide practiced in Washington."

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

"Unlike in the past, where we were unified and drew in allies, currently our own commons seems to be breaking apart."

"To preserve our leadership role, we needed to get our own country's act together first, especially if we were to help others."