Mattis enlisted in the United States Marine Corps Reserves when he was 18 years old in 1969. He was commissioned a second lieutenant after graduating from Central Washington University with a history degree in 1971.
Then-lieutenant colonel Mattis lead the 1st Battalion, 7th Marines (the same unit as legendary Marine Lewis "Chesty" Puller) during combat operations for the liberation of Kuwait from Saddam Hussein.
Dubbed "Task Force Ripper," the unit led the First Marine Division straight into Kuwait City, and took part in the Battle of Kuwait International Airport.
Mattis was promoted to colonel, then brigadier general, and eventually became involved in the early planning and fighting of the Afghanistan War following 9/11.
Under the call sign "CHAOS," an acronym for "Colonel Has An Outstanding Solution," Mattis took a direct approach in combat, and fought on the frontlines on southern Afghanistan with his Marines.
In 2005, he made controversial statements about his time in Afghanistan:
"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. Actually it's quite fun to fight them, you know. It's a hell of a hoot. It's fun to shoot some people. I'll be right up there with you. I like brawling."
Major General Mattis was in charge of the entire First Marine Division during the invasion of Iraq in 2003 — commanding some 20,000 Marines.
Mattis made sure that his Marines achieved their tasks swiftly. When one of his subordinates stalled the Marines' advance at the Battle of Nasiriyah, Mattis relieved him of command and, in an emphatic move, forced him to empty his sidearm of ammunition.
Mattis was a key actor in the first and second Battles of Fallujah, some of the bloodiest actions in the war.
The "Mad Dog's" mission in Iraq continued well past 2003, as he was involved in the stabilization campaign.
After the invasion, Mattis sent the First Marine Division's tanks and artillery back to the US. He then told Iraqi military leaders in a meeting: "I come in peace. I didn't bring artillery. But I'm pleading with you, with tears in my eyes: If you f--- with me, I'll kill you all."
Later on, Mattis said something similar: "We've backed off in good faith to try and give you a chance to straighten this problem out. But I am going to beg with you for a minute. 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."
General Mattis became the source of words to live by in the military.
Here's some of his most notable phrases:
"Be polite, be professional, but have a plan to kill everybody you meet."
"The most important six inches on the battlefield is between your ears."
"Don't create more enemies than you take out by some immoral act."
"You cannot allow any of your people to avoid the brutal facts. If they start living in a dream world, it's going to be bad."
And a rather odd one: "Powerpoint makes us stupid."
Mattis then held other high-level roles, like NATO's Supreme Allied Commander for Transformation, commander of U.S. Joint Forces Command, and the Commander of United States Central Command.
Mattis' commands saw him take direct charge over a number of different operations:
As NATO's Supreme Allied Commander for Transformation, he was tasked with implementing new standards, tactics, and concepts into the Alliance's militaries. He had similar tasks as commander of US Joint Forces Command — though only for the US Military's five branches.
As Commander of United States Central Command (USCC), he oversaw operations in Iraq and Afghanistan, as well as parts of Syria, Iran, and Yemen.
The Obama White House apparently did not place much trust in Mattis, believing him to be too hawkish, especially on Iran. Nevertheless, he served as Commander of USCC until his retirement in 2013.
Just three years after retirement, Mattis became secretary of defense.
General Mattis was newly elected President Donald Trump's candidate to lead the Defense Department. He was confirmed in the Senate by a vote of 98-1.
As secretary of defense, Mattis has focused his efforts on the fight against ISIS, the nuclear threat from North Korea, and stabilizing Afghanistan.
He often seems to care little for politics. During a visit to troops stationed overseas, he told soldiers to "hold the line until our country gets back to understanding and respecting each other and showing it.”
His time in Washington has not eroded his wit or attitude:
In an interview on CBS "Face The Nation," host John Dickerson asked, "What keeps you awake at night?"
The Mad Dog responded almost instantly and in true Mattis fashion:
"Nothing. I keep other people awake at night."
Mattis has become an icon, as this photo that was posted by the Marine Corps Special Operations Command official Facebook shows:
The image also had this written under it:
Hail Mattis.
Full of hate.
Our troops stand with thee.
Blessed art thou among enlisted.
And blessed is the fruit of thy knife hand.
Holy Mattis, father of War,
Pray for us heathen
Now and at the hour of combat.
Amen.