The retired general got the nickname “Mad Dog” after leading combat troops in the Persian Gulf War in 1991.
But the bachelor and avid reader and student of military history also is called “the warrior monk.”
Mattis led the United States Central Command, which oversees military operations in the Middle East and Southwest Asia, from 2010 to 2013.
His tour there was cut short by the Obama administration, which thought he was too hawkish on Iran, according to The New York Times.
In some important policy areas, Mattis differs from Trump, who has been filling the top ranks of his national security team with hard-liners.
Mattis believes, for example, that Trump’s conciliatory statements toward Russia are ill informed. Mattis views with alarm Moscow’s expansionist or bellicose policies in Syria, Ukraine and the Baltics. And he's told Trump that torture doesn't work.
Lawmakers from both parties who’ve met Mattis often have a positive impression of him, says McClatchy.
“I continue to admire his service, knowledge, experience and leadership,” said Rep. Adam Schiff of California, top Democrat on the House intelligence committee.
Mattis would be the first former ranking general to become Defense secretary since George Marshall in 1950-51. Mattis would need a special congressional waiver to serve as defense secretary. He retired from the Marines in 2013, and federal law requires that the Pentagon chief be out of uniform for seven years.
Mattis is given to “pithy aphorisms,” like "Be polite, be professional, but have a plan to kill everybody you meet.” Here are more.