Neither Mattis nor Trump's transition team responded to Military Times' requests for comment.
The 66-year-old retired general, who left active duty in 2013 after reportedly falling from favor with the Obama administration over disagreements about Iran, last served as the head of U.S. Central Command. The post afforded him oversight of all military activity in the Middle East, to include the wars in Afghanistan and Iraq. He will require a waiver from Congress to hold the Pentagon's top post because law mandates a seven-year wait between active-duty service and working as defense secretary, a rule designed to reinforce the concept of civilian control of the military.
Mattis is widely respected on Capitol Hill, and likely won't encounter any difficulty getting confirmed. Senate Armed Services Committee Chairman John McCain, R-Ariz., has praised Mattis as "one of the finest military officers of his generation and an extraordinary leader," and has signaled Senate support for Trump's choice, saying it won't be a problem for Mattis to obtain the waiver he'll need to serve as secretary.
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Trump will look to Mattis, if confirmed, to help navigate a host of global security challenges: Washington's standoff with Russia, China's imperial ambitions, ongoing violence in Africa and the Middle East.
"The Afghan war is not going well," said Peter Bergen, a military analyst and vice president of the New America think tank. "One of the first things the Trump administration needs to do is to figure out its policy there."
Inside the Pentagon, Mattis was known for being assertive with the use of U.S. forces, said Bryan Clark, who was a top aide to Adm. Jon Greenert, the chief of naval operations from 2011 to 2015. Mattis leaned on the Navy to keep two aircraft carriers in the Persian Gulf as a means to counter Iran, creating some strain on the service's budget and resources.
"He pushed a lot more of a hawkish tone towards Iran" said Clark, now an analyst with the Center for Strategic and Budgetary Assessments. "At the time, the Obama administration was trying to use the 'carrot and stick' method to get Iran to the table: The carrot being the nuclear negotiations and the sticks being the stepped up carrier presence. Mattis favored using the stick until the adversary cedes to your wishes — then bring out the carrots."
During his final years of service, Mattis sparred often with Obama’s national security team. As the president moved to set up his nuclear agreement with Iran, Mattis publicly advocated his aggressive approach to confronting the regime he has come to view as the greatest threat to stability in the Middle East. Trump made this a key foreign policy point on the campaign trail, repeatedly blasting Obama and former Secretary of State Hillary Clinton for the nuclear deal.
While China and Russia might call for somewhat different approaches, Clark added that Mattis clearly believes in the power of U.S military muscle to solve problems. "He'll look to be a lot more assertive in our disagreements with other nations," Clark said. "... He postured a large amount of forces with a lot of capabilities in the places they'd want to cause trouble."
The general enjoys a cult-like following among past and present military members — particularly infantry Marines and soldiers — inspired by his swashbuckling rhetoric about the realities of war. He is known by an array of nicknames and military callsigns, including Mad Dog, Chaos and Warrior Monk. The last derives from his bachelor status, a rarity among those who attain four-star status.
Regarded as an intellectual but tough-edged military leader, Mattis is known for his colorful quotes such as: "Be polite, be professional, but have a plan to kill everybody you meet." His 44-year military career, which includes experience on the ground in combat, buoys his credibility. After their initial meeting on Nov. 19, Trump called Mattis "the real deal."
Mattis currently works as a national security fellow at the California-based Hoover Institution. In recent months he has spoken frequently in Washington, D.C., about the need for military leadership and vigilance in an increasingly dangerous world.
The president-elect has indicated a strong desire for a bigger military and fewer spending restrictions. But Trump also has promised a less-confrontational foreign policy strategy, blasting the past two presidents' inclination toward "nation building," calling the approach an unforgivable failure.
It remains to be seen how that syncs with Mattis’ opinions. But in August, the general co-authored a report blasting the last three administrations for a perceived lack of national security vision, saying those leaders have largely ignored threats posed by Russia, China and terrorist groups worldwide.
"If the world feels more dangerous to you, it should," the report states. "We are seeing the results of 20 years of the United States operating unguided by strategy. We have been slow to identify emergent threats and unwilling to prioritize competing interests; we have sent confounding messages to enemies and allies alike. Our country urgently needs to up our game, make common cause with countries that are willing to help repair and sustain the international order that has served the United States and our allies so well."
Trump has said that Mattis may have changed his views on torturing terrorists — even before being formally offered the job. Trump and his surrogates have been advocates for a return to waterboarding and other controversial interrogation techniques, but in an interview with the New York Times on Nov. 22 the president-elect said Mattis made him rethink that position.
Instead, Trump said, Mattis advocated building a relationship with detainees. He told Trump "give me a pack of cigarettes and a couple of beers, and I'll do better," an answer the president-elect called "impressive."
Last spring, a group of Mattis fans unsuccessfully attempted to draft the retired general as an alternative presidential candidate to Trump and Clinton, but Mattis rebuffed the effort. And unlike a number of other high-profile former military leaders, he declined to condemn Trump’s campaign trail rhetoric about attacking the families of terrorists as dangerous and un-American.
In September, Mattis co-authored the book "Warriors & Citizens" which addresses the cultural gap between the military and the civilian population it serves. The results revealed a surprising level of ignorance and unfamiliarity.
His research found that one in three Americans have little or no familiarity with the military, and half of Americans cannot recall socializing with a service member or military spouse within the last year. This may point to at least one of his priorities as defense secretary: bridging the so-called civil-military divide.
"There are many people who do not know if the U.S. Army has 60,000 men or 6 million," Mattis told Military Times when the book was published in September. "They do not have a clue about that.
"America is quite right to be proud of their military, but at the same time there has got to be a sense of common purpose between these two elements. If, in fact, this gap grows and we lose the sense of common purpose, then I think we have a problem."
Mattis espouses and exudes the virtues of military leadership and will bring three "critical elements" to the Pentagon, said retired Adm. James Stavridis, who retired in 2013 as NATO's top commander.
"First a deep profound and unshakable understanding of combat operations from having been there," Stavridis said. "Second an intellectual underpinning that comprehends the deepest elements of military history and strategy. And third an unbounded compassion for the troops that he serves."
With reporting by Jeff Schogol of Marine Corps Times and David Larter of Navy Times. The Associated Press also contributed to this report.
Leo Shane III covers Congress, Veterans Affairs and the White House for Military Times. He can be reached at firstname.lastname@example.org
Leo covers Congress, Veterans Affairs and the White House for Military Times. He has covered Washington, D.C. since 2004, focusing on military personnel and veterans policies. His work has earned numerous honors, including a 2009 Polk award, a 2010 National Headliner Award, the IAVA Leadership in Journalism award and the VFW News Media award.