James Mattis, a Marine Corps general who served as commander of U.S. Central Command before retiring from the military in 2013, was announced as Donald Trump's nominee for Secretary of Defense on December 1. Mattis – whose nicknames include "Mad Dog," "Warrior Monk" and "Chaos" – enlisted in the Marines in 1969 while studying history at Central Washington University. He served in the Gulf War and the wars in Afghanistan and Iraq, where he acquired a reputation as a tough but humane commander. 

Mattis, 66, was confirmed by the Senate on Inauguration Day, January 20; 98 senators voted in favor of his confirmation, while New York Democrat Kirsten Gillibrand voted against it because, a spokesman told the New York Times, "she believes civilian control of the military is fundamental." Mattis had to obtain a waiver from Congress to get the job because the requisite seven years had not elapsed since his retirement from the Marines. His selection marked a move away from civilian control over the Pentagon: Ash Carter, the previous Secretary of Defense, is a physicist who has never served in the military. 

Trump's choice of Mattis signals his administration's focus on the Middle East, North Africa and Central Asia – the region encompassed by Central Command or "Centcom" – in contrast to the Obama administration’s desire to "pivot" to East Asia. 

Mattis is known for his hawkish views on Iran, which he called "the single most enduring threat to stability and peace in the Middle East" and "a revolutionary cause devoted to mayhem" in April. Some reports at the time of his retirement in 2013 suggested he was pushed out by the Obama administration for opposing rapprochement with Iran. At the same time, he resists calls to scrap the nuclear agreement, preferring to "live up to our word." 

As Mattis sees it, the U.S. is becoming "somewhat irrelevant in the Middle East." This implicit criticism of Obama's policies appears to align with Trump's campaign rhetoric, yet Mattis holds a number of views that contrast with Trump's. There is evidence that he has already been able to nudge the president-elect away from some of his more controversial positions.

Speaking to the New York Times on November 22, Trump described meeting Mattis and said he was "very impressed" by the former general’s take on waterboarding. In Trump's retelling, Mattis said, "give me a pack of cigarettes and a couple of beers and I do better with that than I do with torture." Trump, who supported the use of waterboarding on the campaign trail, cautioned that he had not necessarily changed his mind about the interrogation technique.

Mattis would also bring a harder line on Russia to Trump's incoming administration, which has been dogged by allegations that it is too friendly to the Kremlin. Mattis was NATO's Supreme Allied Commander of Transformation and has accused Putin of wanting to "break NATO apart." Trump said in March that the alliance "may be obsolete."

Mattis supports a two-state solution to the Israel-Palestine conflict and he has praised John Kerry, who took over as Secretary of State from Hillary Clinton in 2013, for being "wisely focused like a laser beam" on that goal.  

Since he was a young commissioned officer, Mattis has been known for his intellectual voraciousness, symbolized by the copy of Marcus Aurelius' "Meditations" he brought on every deployment. He has advocated humane, culturally aware treatment of civilians in war zones, writing to members of the 1st Marine Division in the run-up to the Iraq War, "While we will move swiftly and aggressively against those who resist, we will treat all others with decency, demonstrating chivalry and soldierly compassion for people who have endured a lifetime under Saddam’s oppression."

At the same time, he has come in for criticism for his blunt descriptions of warfare – "it's a hell of a hoot, it's fun to shoot some people" – and for his role in authorizing the so-called "Mukaradeeb wedding party massacre," a 2004 incident in which American air and ground forces killed over 40 civilians at what reports described as a wedding party. Mattis and other military leaders defended the operation, arguing that the gathering was not a wedding and implying that many of the guests were militants. 

Prior to his resignation on January 5, Mattis was a member of the board of Theranos Inc., the troubled medical device company founded by Elizabeth Holmes. He remained on the board of General Dynamics Corp. (GD), an aerospace and defense firm, but said he would resign if confirmed.

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