AP Donald Trump at a rally with James Mattis, his pick for 
Before his hearing, Congress first had to grant Mattis a waiver allowing him to serve in civillian government less than seven years after retiring from the military, as statute dictates.
Mattis will take over for current Secretary of Defense Ash Carter.
Mattis enjoyed a relatively smooth confirmation process when speaking before congress, with many observers noting that his answers broke from some of Trump's positions.
The US is "under the biggest attack since World War II," Mattis noted during his hearing, with threats "from Russia, from terrorist groups, and with what China is doing in the South China Sea."
"I have very modest expectations about areas of cooperation with Mr. Putin," Mattis added. Trump has expressed optimism for relations with Putin.
Additionally, Mattis admitted that Obama's signature foreign policy achievement, the Iran deal, is "not a deal I would have signed." However, Mattis concluded that "when America gives her word, we have to live up to it and work with our allies," and that he would not rip up the deal as Trump has often proposed.