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The retired four-star general gave frank and concise answers on everything from cybersecurity policy to what he expects will be the biggest threats to the United States.
Shortly afterward, he was approved for a waiver for the requirement of having a seven-year gap between being active-duty in the military and serving in the civilian role at the Pentagon.
Mattis thinks the US needs to be more aggressive going after ISIS
When asked whether the US can confront the terror group in its capital of Raqqa, Mattis said he believed the US could, but he added that the anti-ISIS strategy needed to be reviewed and "energized on a more aggressive timeline."
He told members that "we have to deliver a very hard blow against ISIS in the Middle East so there is no sense of invulnerability or invincibility there." For the US, according to Mattis, that means attacking ISIS' main areas of strength so they cannot pop up elsewhere.
He mentioned Russia as the biggest threat
Despite President-elect Trump's restraint on calling out Russian aggression and cyberwarfare, Mattis didn't pull punches in his assessment of Moscow.
"Since Yalta, we have a long list of times that we've tried to engage positively with Russia," Mattis said. "We have a relatively short list of successes in that regard." He praised NATO and its effectiveness, and added that Russian President Vladimir Putin was "trying to break" that alliance.
In some areas Russia and the US can work together, but in many others, Mattis said, Putin remained a strategic competitor or an outright adversary.
"I have very modest expectations about areas of cooperation with Mr. Putin," he said.
Mattis says he wouldn't roll back the repeal of Don't Ask, Don't Tell or the exclusion of women in combat roles
In the past, Mattis has not really been a fan of women being integrated into combat roles, such as infantry. He was asked about this repeatedly - at times having his speeches quoted to him - and asked whether he would change the 2013 policy change.
"I've never come into any job with an agenda, a pre-formed agenda of changing anything," he said. "I assume the people before me deserve respect for the decisions they've made."
That answer did not satisfy Sen. Kirsten Gillibrand (D-N.Y.), however, and she continued to press him. In the end, Mattis told her: "I have no plan to oppose women in any aspect of our military. In 2003, I had hundreds of Marines who happened to be women serving in my 23,000 person division ... I put them right on the front lines with everyone else."
He was also asked about protections for LGBTQ service members, and he had a very frank answer to that. "Frankly, senator, I've never cared about two consenting adults and who they go to bed with."
He says the Iran deal isn't perfect, but it should remain intact
Mattis called the Iran deal an "imperfect" one, but still supported the US keeping its end of the bargain. The answer was a break from the President-elect, who has promised to "rip up" the deal with Tehran.
"I think it is an imperfect arms control agreement - it's not a friendship treaty," Mattis said. "But when America gives her word, we have to live up to it and work with our allies."
Later, he said, "It's not a deal I would have signed."
Mattis says cyberwar is a big problem that still has no clear doctrine in place
Mattis was asked interesting questions on cyberwarfare, which were especially pertinent in the wake of Russian hacks of Democratic party officials and their affect on the presidential election. Unfortunately, he said he did not believe the US has anything resembling a sophisticated cyber doctrine.
In other words, there is no strategy in place for the US to respond to cyber attacks, like there is for other physical examples, such as a nuclear strike or an attack on a NATO ally.
Mattis said there needs to be a comprehensive plan developed to address this shortfall, because "cyber cuts across everything we do today."
He added: "Because of the cyber domain, it's not something the military can do in isolation."