The ex-general has been criticised for saying he enjoys shooting people, and serving on the board of a troubled biotech company.
Controversial retired general James Mattis has been named by Donald Trump as his choice for defence secretary.
The 66-year-old retired from the Marine Corps in 2013, having served as the commander of the US Central Command.
Nicknamed "Mad Dog", he was referred to by President-elect Mr Trump as "a true general's general".
But, while his service record is mostly respected, there are concerns about his lack of experience in diplomacy.
There is also concern about some of his other activities, including serving on the board of troubled biotech firm Theranos.
The Washington Post reported that Theranos founder Elizabeth Holmes had tried to get him to intervene after a Pentagon official questioned the company's selling of its technology without approval from the Food and Drug Administration.
Theranos promised new-age blood-testing technology but had to invalidate two years of results after the reliability of its methods were questioned.
Mr Mattis also said in 2005 that it was "fun to shoot some people".
He told a forum in San Diego: "Actually, it's a lot of fun to fight. You know, it's a hell of a hoot. It's fun to shoot some people.I'll be right up front with you, I like brawling."
Mr Mattis added: "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."
For Mr Mattis to be confirmed, a new law would have to be approved to bypass current legislation barring military officers from being defence secretary within seven years of leaving active duty.
Controversial retired general James Mattis has been named by Donald Trump as his choice for defence secretary.
The 66-year-old retired from the Marine Corps in 2013, having served as the commander of the US Central Command.
Nicknamed "Mad Dog", he was referred to by President-elect Mr Trump as "a true general's general".
But, while his service record is mostly respected, there are concerns about his lack of experience in diplomacy.
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| General Mattis has an admirable service record but there are fears about his diplomatic ability |
The Washington Post reported that Theranos founder Elizabeth Holmes had tried to get him to intervene after a Pentagon official questioned the company's selling of its technology without approval from the Food and Drug Administration.
Theranos promised new-age blood-testing technology but had to invalidate two years of results after the reliability of its methods were questioned.
Mr Mattis also said in 2005 that it was "fun to shoot some people".
He told a forum in San Diego: "Actually, it's a lot of fun to fight. You know, it's a hell of a hoot. It's fun to shoot some people.I'll be right up front with you, I like brawling."
Mr Mattis added: "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."
For Mr Mattis to be confirmed, a new law would have to be approved to bypass current legislation barring military officers from being defence secretary within seven years of leaving active duty.