WASHINGTON (Reuters) - U.S. President-elect Donald Trump said on Thursday night he would nominate retired Marine Corps General James Mattis, known as “Mad Dog” and renowned for his tough talk and battlefield experience in Iraq and Afghanistan, to lead the Pentagon.

“We are going to appoint ‘Mad Dog’ Mattis as our secretary of defense,” Trump told a rally in Cincinnati. He said the formal announcement would be made on Monday.

The choice of a seasoned military strategist would be another indication that Trump, a Republican, intends to steer U.S. foreign policy away from Democratic President Barack Obama’s increased reliance on U.S. allies to fight Islamist militants and to help deter Russian and Chinese aggression in Europe and Asia.

Mattis is a revered figure in the Marine Corps and known for his distrust of Iran.

The Washington Post and CNN reported earlier that Trump had chosen Mattis, but Trump spokesman Jason Miller said earlier on Twitter that “no decision has been made yet with regard to Secretary of Defense.”

While the nomination of the 66-year-old Mattis would likely be popular among U.S. forces, it would have to clear a bureaucratic hurdle.

Because he retired only in 2013, Mattis would need the U.S. Congress to waive a requirement that a defense secretary be a civilian for at least seven years before taking the top job at the Pentagon. His impressive combat record, however, may deter some Senate Democrats from trying to block his nomination.

Trump has described Mattis as “a true general’s general.”

The New York real estate magnate famously asserted last year: “I know more about ISIS than the generals do.”

Mattis, whose past assignments include leading Central Command, which oversees U.S. military operations in the Middle East and South Asia, is known for his colorful expressions that unashamedly embrace the job of the U.S. armed forces: fighting wars.

In one famous line in 2003 attributed to Mattis, the general told Marines in Iraq: “Be polite, be professional, but have a plan to kill everybody you meet.”

TOUGH TALK

In a 2016 question-and-answer session, Mattis appeared to be moved by a Marine’s question about how far out he could inflict casualties with his knife hand, known as a “kill-casualty radius.”

“Once you get to be a high-ranking officer, the kill-casualty radius is whatever your Marines make it, and by the time I got up to the senior ranks it was hundreds of miles,” he said in a video for the Marine Corps.

Still, such tough talk has gotten him in hot water. He was once rebuked for saying in 2005 that “it’s fun to shoot some people.”

His talk, however, belies a more thoughtful side. Mattis once said the most important 6 inches in a combat zone was “between your ears.”

Now a fellow at Stanford University’s Hoover Institution, Mattis is also a scholar who was praised by then-U.S. Defense Secretary Robert Gates in 2010 as one of the country’s great strategic thinkers.

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Mattis reads avidly, frequently quotes history and is proud that he grew up with a large library and no television.

After meeting Mattis on Nov. 19, Trump described him as a strong, dignified man who persuasively argued against waterboarding, an interrogation tactic that involves pouring water over someone’s face to simulate drowning.

Trump had promised during the campaign he would not only revive use of waterboarding, which is widely regarded as torture and was banned under President Barack Obama, but bring back “a hell of a lot worse” if elected.

“(Mattis) said: ‘I’ve always found, give me a pack of cigarettes and a couple of beers and I do better with that than I do with torture.’ And I was very impressed by that answer,” Trump told The New York Times.

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WARY OF IRAN

The Senate Armed Services Committee will consider Mattis’ nomination. In a statement on Thursday night, its chairman, Republican John McCain, called him “one of the finest military officers of his generation and an extraordinary leader.”

Mattis would be the first former U.S. general to become defense secretary since George C. Marshall took the job in 1950.

The decision adds to Trump’s national security team another Pentagon veteran who served during the Obama administration but often had a testy relationship with it.

Officials who knew him before he retired in 2013 said Mattis clashed with top administration officials when he headed Central Command over his desire to better prepare for potential threats from Iran and to win more resources for Afghanistan.

Trump has given the job of national security adviser to Michael Flynn, a retired three-star Army general who was pushed out of the top job at the Defense Intelligence Agency in 2014 by Obama’s administration.

Flynn was fiercely critical of Obama during the 2016 campaign, adopting much of Trump’s rhetoric.

Along with Flynn and Trump’s choice for CIA director, U.S. Representative Mike Pompeo, Mattis has been critical of the deal to curb Iran’s suspected nuclear weapons program, saying the threat from Tehran should outrank more immediate concerns about Islamic State or al Qaeda.

“The Iranian regime, in my mind, is the single most enduring threat to stability and peace in the Middle East,” Mattis said.

Speaking about the Iranian nuclear deal, Mattis said: “Hoping that Iran is on the cusp of becoming a responsible, modern nation is a bridge too far.”

If Mattis wins Senate confirmation, he will work side by side with another Marine - General Joseph Dunford, who is chairman of the Joint Chiefs of Staff.

Having two Marine generals in those top jobs would be highly unusual for a service that prides itself on being the most elite U.S. fighting force. It would also raise questions about how Mattis and Dunford might divide up tasks.

Both Dunford and Mattis share battlefield experience, including in Iraq. In 2003, Mattis led the 1st Marine Division during the U.S.-led invasion of Iraq.

He has said one of the toughest things he had to do was oversee the retreat of his forces from the city of Falluja in 2004, something he feared would hurt morale, but did not.

“We just don’t take refuge in self-pity or any of that kind of stuff. And so as a result, the Marine Corps remains a very feared organization in this world. As it should be,” he said.