Mattis, 66, is a Marine Corps general who retired in 2013 after serving as the commander of the U.S. Central Command.

His selection raises questions about increased military influence in a job designed to insure civilian control of the armed forces. The concerns revolve around whether a recently retired service member would rely more on military solutions to international problems, rather than take a broader, more diplomatic approach.

For Mattis to be confirmed, Congress would first have to approve legislation bypassing a law that bars retired military officers from becoming defense secretary within seven years of leaving active duty.

Mattis has a reputation as a battle-hardened, tough-talking Marine who was entrusted with some of the most challenging commands in the U.S. military. In a tweet last month, Trump referred to Mattis by his nickname “Mad Dog” and described him as “A true General’s General!”

Mattis would be only the second retired general to serve as defense secretary, the first being George C. Marshall in 1950-51 during the Korean War. Marshall was a much different figure, having previously served as U.S. secretary of state and playing a key role in creating closer ties with Western Europe after World War II.

The only previous exception to the law requiring a gap after military service was for Marshall.

Although his record in combat and his credentials as a senior commander are widely admired, Mattis has little experience in the diplomatic aspects of the job of secretary of defense.

Richard Fontaine, president of the Center for a New American Security, described Mattis as a defense intellectual and as a military leader who distinguished himself in combat.

“He knows the Middle East, South Asia, NATO and other areas and has evinced both a nuanced approach to the wars we’re in and an appreciation for the importance of allies,” Fontaine said in an email exchange. “If he were to get the nomination, I suspect that he could attract a number of very talented people to work with him.”

But Mattis hasn’t been immune to controversy. He was criticized for remarking in 2005 that he enjoyed shooting people. He also drew more recent scrutiny for his involvement with the embattled biotech company Theranos, where he serves on the board.

Born in Pullman, Washington, Mattis enlisted in the Marine Corps in 1969, later earning a history degree from Central Washington University. He was commissioned as an officer in 1972. As a lieutenant colonel, Mattis led an assault battalion into Kuwait during the first U.S. war with Iraq in 1991.

As head of the Central Command from 2010 until his retirement in 2013, he was in charge of both the wars in Iraq and Afghanistan.

In the aftermath of the 9/11 attacks, Mattis commanded the Marines who launched an early amphibious assault into Afghanistan and established a U.S. foothold in the Taliban heartland.

As the first wave of Marines moved toward Kandahar, Mattis declared that, “The Marines have landed, and now we own a piece of Afghanistan.”

Two years later, he helped lead the invasion into Iraq in 2003 as the two-star commander of the 1st Marine Division.

In 2005, he raised eyebrows when he told a San Diego forum that it was “fun to shoot some people.”

According to a recording of his remarks, Mattis said, “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.”

He added, “You go into Afghanistan, you got guys who slap women around for five years because they didn’t wear a veil,” Mattis continued. “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.”

Mattis was counselled to choose his words more carefully.

A year later, Mattis came under scrutiny during one of the more high-profile criminal investigations of the Iraq war, the shooting deaths of 24 Iraqis by Marines.

The Iraqis, who included unarmed women and children, were killed by Marines in the town of Haditha after one of their comrades was killed by a roadside bomb. Eight Marines were charged in connection with the killings — four enlisted men were charged with unpremeditated murder and four officers who weren’t there at the time were accused of failures in investigating and reporting the deaths.

As commander of the accused Marines’ parent unit, the 1st Marine Expeditionary Force, Mattis ultimately dismissed charges against most of the Marines.

As a top Marine general, Mattis pushed for the military to adopt blood-testing technology developed by Theranos.

As reported by The Washington Post, Mattis first met Theranos founder Elizabeth A. Holmes in 2011. A year later, according to emails obtained by The Post, Holmes used her connection to Mattis to pressure him to intervene after a Pentagon official raised concerns that the company was distributing its technology without approval by the Food and Drug Administration.

The emails show within hours after Holmes asked Mattis for help, he forwarded her email to other military officials asking them,”how do we overcome this new obstacle.”

Mattis joined the Theranos board the same year he retired. The company, which raised hundreds of millions of dollars on the promise of breakthrough blood-testing technology, was forced to invalidate two years of patients’ test results after the reliability of its proprietary blood-testing machinery was questioned by internal and government whistleblowers and investigative reporting by The Wall Street Journal.