Retired U.S. Marine Corps General James Mattis is President Trump's Secretary of Defense. Here's what you need to know about "Mad Dog Mattis." (Jenny Starrs/The Washington Post)

Marine Lance Cpl. Andrew Gitto was less than two weeks removed from his 20th birthday when a Taliban sniper hit him two times at a checkpoint in Marja, Afghanistan, in April 2010. One round grazed his left hand and lodged in his shoulder, while the second buried into his lung, collapsing it.

Six days later, Marine Gen. James N. Mattis, then the head of U.S. Joint Forces Command, was standing at the end of his hospital bed in Bethesda, Md., talking quietly with Gitto’s mother, JoAnn. Gitto, now 26 and a sheriff’s officer in New Jersey, doesn’t remember much of his drug-addled 14-day hospital stay, but he recalls Mattis visiting and giving him a signed copy of a Steven Pressfield novel, “Gates of Fire.”

“He didn’t treat me like a guy who’s been shot,” Gitto said. “He talked to me like we’ve known each other for years. He shook my hand, looked me in the eye and told me heal back up and head back. And that’s what I did.”

President-elect Donald Trump announced Thursday that Mattis, 66, will be nominated as his secretary of defense.

Gitto’s story is one of many about Mattis that, along with his bluntness and success on the battlefield, have made him perhaps the most popular senior officer of the modern generation with his own troops.

Nathaniel Fick, a former Marine officer who served under Mattis in combat, said that while he doesn’t want to be in a society where “the guys with the guns” make all the decisions, the retired general could be valuable in his new role.

“The thing about Mattis I believe to my core is that he will not commit forces in harm’s way unless he believes that U.S. interests are at stake and there is a plan to win,” said Fick, now the chief executive officer of the cybersecurity firm Endgame Inc. “It looks like we’re going to have a lot of people in the Situation Room who have never been in the Situation Room before. And with Mattis, you have someone who has been — and also has been on the front lines.”

Some critics, however, have argued the appointment give military officers too much sway over foreign policy.

Loren DeJonge Schulman, a former Pentagon and National Security Council staff member, said that she has no concerns about Mattis individually, she wonders what his appointment could do to the role of Pentagon chief.

“He is being praised as a warfighter, and that’s a great experience to be able to bring to the Office Secretary of Defense, but that is in no way the role of the office,” said Schulman, now the deputy director of studies at the Center for a New American Security. “It would be extremely damaging of the [civilian-military] dynamic if that became the perception of the role.”

Mattis, who declined to comment for this story, led combat troops in the early salvos of the war in Afghanistan in late 2001 and Iraq in spring 2003, before returning to Iraq a second time to lead bloody street fighting in the city of Fallujah in 2004. He continued to ascend through the ranks afterward, becoming one of the military’s top strategists before retiring as the chief of U.S. Central Command in 2013.

His call sign: “Chaos.”

Gritty stories about and quotes from Mattis have become part of military lore often getting passed around on the Internet in the form of colorful memes and lists. Marine Corps Special Operations Command (MARSOC) even got in on the act Thursday night, posting on their official Facebook page an often-shared tribute to Mattis that depicts him as “Saint Mattis of Quantico, Patron Saint of Chaos,” with a grenade in one hand and a knife in the other. It was later taken offline.

Rep. Ryan Zinke (R-Mont.), a Trump supporter and former Navy SEAL,  said that Mattis will be faced with reforming a Defense Department that has become a “labyrinth of red tape.” Zinke first met Mattis in Fallujah in 2004, and said that he was a straight shooter who was not hesitant to make unpopular or difficult decisions.

“He is not politically correct, which is what I think we need,” Zinke said. “But he fully understands the consequences of war and the tremendous cost that is likely to be incurred by going into conflict.”

One issue has emerged that would not be faced by most defense secretary candidates: Past decisions on the battlefield. A retired Special Forces officer, Jason Amerine, wrote a long Facebook post Friday in which he accused Mattis of leaving his men to die in Afghanistan in December 2001, when the general was a one-star commander overseeing an amphibious task force in Kandahar province. Mattis’s unit was nearby when Amerine’s was hit in a friendly-fire bombing, and declined for hours to send helicopters to rescue those who were wounded, Amerine said.

“Thanks to Facebook mob mentality, there has been a revisionist history of the man who left us to die but is now compared to Patton due to his juvenile quotes that demean all of us as service members,” Amerine wrote in the post, which went viral.

But Jeremiah Workman, a Marine veteran who served in Iraq, said that not only do the general’s past successes speak for themselves, he has gone out of his way to assist those who struggle after coming home from war. Workman recalled receiving a surprise phone call from Mattis when he was a sergeant in 2007 at Quantico, Va. Workman had received the Navy Cross for valor in Iraq but later lost his job as a drill instructor at Parris Island, S.C., while coping with post-traumatic stress.

“When I think of a leader, I think of genuine care and concern, and that’s him,” said Workman, who is now a Department of Veterans Affairs employee at Quantico. “Just the fact that General Mattis was a three-star general who was reaching out to a sergeant in the Marine Corps who was hurting and needed help, that speaks volumes for him as a person.”

Related coverage:
How Trump picking Mattis breaks with 65 years of history

Here are the people whose names have been floated for Trump’s Cabinet

Pentagon and intelligence community chiefs have urged Obama to remove the head of the NSA