After Congress passed a waiver allowing a general into the post and should the Senate confirm Mattis, he will bring his considerable experience to help lead a nation long at war.
Trump's choice of Mattis brings a complex personality to the table. As CNN national security analyst Peter Bergen put it
, on one hand, he has espoused hawkish views on Iran and his fierce language recalls Trump's apparent hero Gen. George Patton. One the other, Mattis is an intellectual whose views on torture (against) and warfare (studied) may clash with his commander-in-chief.
A retired four star general, Mattis, a bachelor, is a so-called "warrior monk," married to the Marines. He served in the first Gulf War and in Afghanistan. He once led the all-important US Central Command, which is in charge for the US military in the Middle East. He was also Supreme Allied Commander of NATO.
If confirmed, Mattis would be the highest-ranking military official ever to lead the Pentagon.
Additionally, he represents some of what Trump was selling on the campaign trail. He's a fierce fighter and wary, to say the least, of Iran. Like Trump, he has looked back on the Iraq War and called it a "mistake," according to audio obtained
by The Intercept.
What do Mattis' critics say?
Some have latched onto Mattis' blunt statements
about violence in war as inappropriate and inhumane. Others have called for Trump to pick a defense secretary who would temper tensions in the world, especially with Iran.
Some of his wartime operational decisions
have also come under scrutiny
. A former special forces officer accused Mattis of being slow to launch a rescue mission in a 2011 book
What made Mattis who he is?
The Marine Corps, decades of war and a lifetime of study have built Mattis into the man who stands on the cusp of service yet again.
Outside of his experience in warfare, he is an obsessive reader and an accomplished writer in his field. Along with retired Gen. David Petraeus, Mattis literally wrote the book on counterinsurgency in the lead up to the US' surge in Iraq.
Three years after President Barack Obama tapped him to replace Petraeus as the head of US Central Command, Mattis retired and went on to co-author a book outlining his beliefs. Many of his writings and statements made known his differences with the Obama administration, particularly on Iran and Afghanistan.
He's known widely for sayings that are frank about violence.
"Be polite, be professional, but have a plan to kill everybody you meet," he would tell his Marines in Iraq, according to the writer Thomas Ricks in his book Fiasco. Or, "It's fun to shoot some people," he said in 2005
, drawing criticism.
These sayings have helped earn him the nickname "Mad Dog."
Additionally, and totally separate from his military career, Mattis has ties
, a scandal-plagued healthcare technology company.