Donald Trump's pick for defence secretary is a hard-charging former marine general nicknamed Mad Dog who, despite his rank, regularly rode convoys into combat during the Iraq invasion.

James Mattis, former commander of U.S. Central Command, also has a gift for boot-stomping aphorisms.

"Be polite, be professional, but have a plan to kill everybody you meet," Mattis reportedly told marines stationed in Iraq.

But don't be misled by the colourful myth-making, says former U.S. Army general Barry McCaffrey. Mattis reportedly did not care for the Mad Dog label, a moniker that McCaffrey says belies a careful and studied military mind.

"The least of our worries ought to be that Mattis is a 'Mad Dog,'" says the Seattle-based four-star officer, who applauds Mattis's nomination as a buffer against what he calls an "ignorant and erratic" Trump brain trust.

As much as the appointment should "scare the daylights" out of enemies of the U.S., McCaffrey says, expect the 66-year-old to be a "moderating voice" in the West Wing if he's confirmed.

"We'll have a steady, knowledgeable hand at defence to counter the president-elect's lack of experience and impulsiveness."

U.S. soldiers take part in a live shooting exercise during U.S. operation "Atlantic Resolve" in Adazi, Latvia, earlier this year. If confirmed, Mattis would have the authority to send troops into battle. (Ints Kalnins/Reuters)

Tapping the 41-year Marine Corps veteran brings serious heft to security matters, military scholars say — particularly with regard to Mideast affairs, U.S.-Russia relations, recognizing the value of NATO and rethinking Trump's pledge to "rip up" the Iran nuclear deal.

To McCaffrey, Trump's pick is a "warrior-monk intellectual" with astute knowledge of military history and tactics that will be invaluable in the new administration.

"If I was a jihadist, I'd be digging a tunnel down into the basement and be 60 days into it when they take office."

Could sway Trump against torture

Perhaps more importantly, Mattis seems to wield influence on the soon-to-be Commander-in-Chief, notes Charles Dunlap, a retired U.S. Air Force major-general who knows Mattis personally.

"One example is how, after one relatively short meeting, he had Trump rethinking his controversial views on torture," says Dunlap, a Duke University law professor.

The defence nominee's conversation in Trump Tower last month "surprised" Trump, who told reporters that Mattis's objection to waterboarding gave him pause.

That's noteworthy because they diverge on other issues in which Mattis might hold sway.

General James "Mad Dog" Mattis, who is being considered for Secretary of Defense, was very impressive yesterday. A true General's General! —@realDonaldTrump

If confirmed, Mattis would likely dissuade Trump from engaging too warmly with Russian President Vladimir Putin, whom Trump has praised as a better leader than U.S. President Barack Obama. (Mattis's would-be underling, national security adviser Michael Flynn, has enjoyed cozier ties with Russia and last year accepted payment to participate in an event in Moscow, where he dined next to Putin.)

Although Mattis is hawkish on Iran, calling it the greatest threat to peace in the Middle East, he's also on the record arguing there's no sensible way for the U.S. to unilaterally scrap the Iran nuclear accord, as Trump has vowed he would.

As for NATO, Trump has indicated he might want to withdraw U.S. military aid from the 28-member alliance.

Mattis was a former commander of a major NATO strategic command and has thought deeply "about how American forces should work jointly with different armed services … particularly NATO," says Richard Kohn, a military historian at the University of North Carolina.

Lawmakers across the aisle are, for the most part, supportive of the Mattis pick. The loudest dissenting voice from Congress has been from New York senator Kirsten Gillibrand.

One bureaucratic hurdle remains, however.

Historically, in order to prevent against military dictatorship or the possibility of a coup, the framers of the U.S. Constitution baked in provisions to ensure civilians control the military. That means selecting Mattis would require a waiver, or congressional approval, to change the law stipulating a secretary of defence must have left active duty at least seven years ago.

While I respect Gen Mattis’s service, I'll oppose a waiver-civilian control of our military is a fundamental principle of American democracy —@SenGillibrand

'Not a bloodthirsty man'

Mattis has only been out three years, retiring from Central Command in 2013 after reportedly being sacked by Obama for his aggressive line on Iran.

Kohn, who specializes in civilian-military relations, notes that as defence secretary, Mattis would be the only person aside from the Commander-in-Chief with the authority to send troops into battle.

Mattis, shown in a 2011 file photo, spent more than four decades in the U.S. military. If confirmed, he's expected to have a strong voice in the Trump administration. (Matt Dunham/Associated Press, Chris Bergin/Reuters)

"This is not a bloodthirsty man," Kohn says. "He's a fact-based-reality-keen person who tries to meet the world as it is…We can trust him with the care and lives of our children, of our grandchildren."

That said, he doesn't think Mattis will run into problems getting a congressional waiver.

Neither does Dunlap, who says he expects the Senate confirmation to go smoothly.

The last time Congress passed a special act was in 1950 in order to allow retired general George C. Marshall to step into the defence secretary role at the outbreak of the Korean War.

Learning curve

There will be something of a learning curve if Mattis ends up running the Armed Forces, an organization comprising some 2.2 million men and women in the active guard and reserve forces, as well as 25,000 Pentagon staffers. He will be green on the diplomatic aspects of the post.

Retired Army general Barry McCaffrey, shown in a 2009 file photo on Capitol Hill, applauds the nomination of James Mattis for U.S. Secretary of Defense. McCaffrey has described Mattis as a 'warrior-monk intellectual.' (Haraz N. Ghanbari/Associated Press)

Bureaucratic paperwork, managing the media and dealing with the political arena will be newer to him, says McCaffrey, the four-star Army general in Seattle.

For all his battlefield experience, "one thing [Mattis] doesn't have is a life history of dealing with large managerial responsibilities in this way," he says.

"So it's an unusual pick, but of course we've also got an unusual president-elect."