Image copyright AP

"Be polite, be professional, but have a plan to kill everybody you meet."

That's the stark advice given to Marines in Iraq in 2003 by retired four-star General James Mattis, the man President-elect Donald Trump has named as his defence secretary.

Gen Mattis, who is dubbed "Mad Dog", is known as much for his memorable turn of phrase as his uncompromising approach to America's enemies.

The 66-year-old's quotes, or #mattisisms, have been faithfully shared online by veterans who revere his candour and leadership, much of it served at the frontline of battle.

After meeting him in November, Mr Trump said Gen Mattis - who had a 44-year career with the Marine Corps - was "the real deal" and "a true General's General".

'Warrior Monk'

While rising through the ranks, Gen Mattis also earned the nickname "Warrior Monk" because he never married or had children.

Image copyright Getty Images Image caption President-elect Donald Trump stands with General Mattis after the two met at Trump International Golf Club in New Jersey

He retired in 2013 after serving as head of US Central Command (Centcom), the American military's wing in charge of all its Middle East forces.

Gen Mattis was an outspoken critic of the Obama administration's Middle East policy, specifically its views on Iran.

He has called Iran "the single most enduring threat to stability and peace in the Middle East".

Before he was promoted to lead Centcom in 2010, Gen Mattis was appointed as the head of US Joint Forces Command and Nato Supreme Allied Commander in 2007.

Media playback is unsupported on your device Media caption Donald Trump has named General James Mattis as his secretary for defence

Gen James Mattis, in his own words

"I don't lose any sleep at night over the potential for failure. I cannot even spell the word." "The first time you blow someone away is not an insignificant event. That said, there are some assholes in the world that just need to be shot." "I come in peace. I didn't bring artillery. But I'm pleading with you, with tears in my eyes: If you [expletive] with me, I'll kill you all." "The most important six inches on the battlefield is between your ears." "There is nothing better than getting shot at and missed. It's really great." "I'm going to plead with you, do not cross us. Because if you do, the survivors will write about what we do here for 10,000 years."

(Source: San Diego Union Tribune)

Gen Mattis led an assault battalion during the first Gulf war in 1991 and commanded a task force into southern Afghanistan in 2001.

He also helmed a Marine division at the time of the Iraq invasion in 2003 and played a key role a year later in the Battle of Falluja.

Gen Mattis co-authored a counterinsurgency manual, which was credited for easing sectarian violence in Iraq before the US withdrawal in December 2011.

Image copyright Getty Images Image caption General Mattis with Iraqi General Mohammed Latif, head of the Fallujah Brigade, in 2004

But his refusal to mince words has been both celebrated and censured.

In 2005, Gen Mattis came under fire for comments made while talking to service members in San Diego.

"It's fun to shoot some people. I'll be right up there with you. I like brawling," he said at a panel discussion.

"You go into Afghanistan, you got guys who slap women around for five years because they didn't wear a veil.

"You know, guys like that ain't got no manhood left anyway. So it's a hell of a lot of fun to shoot them."

The Marine Corps said at the time the general had been spoken to about his remarks and agreed "he should have chosen his words more carefully".

'No going back'

He defended a military strike which killed 42 people in Iraq in 2004, which the US said targeted a militant safe house, but survivors and many reports said was a wedding party.

"Bad things happen in wars. I don't have to apologise for the conduct of my men," he was quoted as saying at the time.

Aside from his strong turns-of-phrase, he reads widely, and his foreign policy beliefs differ in some areas to those of Mr Trump.

Donald Trump told the New York Times he was "surprised" to discover Gen Mattis does not favour waterboarding. The president-elect quoted him as saying he'd "never found it to be useful" and could "do better" with "a couple of beers and a pack of cigarettes".

Gen Mattis says Iran is the "single most enduring threat" in the Middle East - bigger than al-Qaeda, Syria and so-called Islamic State.

He's been strongly critical of the Iran nuclear deal, but in a speech to the Center for Strategic and International Studies in April, he said there's "no going back" on the agreement - which Mr Trump has said he wants to "dismantle".

The US would be alone globally if it did try to tear up or pull out of the deal, he argued, and should instead focus on enforcing it and being prepared for further threats.

'Strategy-free mode'

He has warned that Washington has not taken seriously enough Russia's military moves against its neighbours - annexing Crimea and backing separatists in Ukraine - and also criticised Mr Trump's view that Nato is "obsolete" as "kooky", and challenged his suggestion that some Nato allies are not paying their fair share.

But like Mr Trump, he has cautioned about giving information about strategy against enemies such as IS - specifically in saying there will be no "boots on the ground".

In general, on foreign policy, in the CSIS speech in April he said "the next president is going to inherit a mess", with the US on "strategy-free mode" and "shifting our focus from one region or sub-region to another".

A retired officer is required to be out of uniform for at least seven years before he or she can serve as defence secretary.

Gen Mattis, who has only been retired for three years, required a formal waiver from the Republican-controlled Congress to take up the role.

His confirmation made him the country's second retired general to serve as Pentagon chief.

Army General George Marshall received a congressional waiver to serve as President Harry Truman's defence secretary in 1950. He had previously served as President Truman's Secretary of State.

Gen Mattis also drew scrutiny for his membership on the board of directors at discredited blood testing firm Theranos.

Before leaving the military, Gen Mattis lobbied to have Theranos' products approved for use on the battlefield.

He joined the firm's board after leaving the US military in 2013 and Theranos' technology was later found to be fraudulent.