Story highlights Mattis is revered by troops for his leadership and quips

He clashed with the Obama administration on Iran and was relieved of his Centcom post

(CNN) The next possible US secretary of defense went by the military call sign "Chaos."

Revered by his troops as a "warrior monk" with a knack for hard-edged quips, retired Marine Gen. James Mattis led troops in Afghanistan in 2001, won laurels for leadership in one of the bloodiest battles of the Iraq War and most recently headed US Central Command, perhaps the military's most complicated and challenging post.

Now, Mattis faces an entirely different kind of fight.

As President-elect Donald Trump prepares to formally nominate the former four-star to head the Pentagon, some Democrats are signaling his confirmation might not be entirely easy. Some observers question whether Mattis' battlefield experience prepares him for the very different task of running an enormous bureaucracy, while senior lawmakers worry about what the 66-year-old's nomination means for maintaining civilian control of the military.

Republicans issued glowing testimonials to Mattis and his career. California Rep. Devin Nunes, who heads the House Permanent Select Committee on Intelligence, said he could think of "no better candidate to lead America's military in our long fight against jihadism and countering other pressing threats."

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