Mattis, 66, also addressed the recent attacks on U.S. intelligence agencies at his confirmation hearing, noting that he has a "very very high degree of confidence" in them based on his "close relationship with the intelligence community." According to Mattis, he interacted with the nation's spy agencies almost daily during his time in the military.
While Trump has often said the Iranian deal should be scuttled, Mattis said the United States has to live up to its agreement. "When America gives its word we have to live up to it and work with our allies," he testified.
He also told the committee that he "would not have taken this job if I didn't believe the president-elect would also be open to my input on this or any other matter."
The 'Most Successful Military Alliance'When asked about the country's membership in NATO, which Trump has repeatedly criticized, Mattis called it the "most successful military alliance probably in modern world history, maybe ever."
He saw the United States "maintaining the strongest possible relationship" with NATO. "My view is that nations with allies thrive and nations without allies don't," he argued.
He said he has spoken with Trump about the issue and found him "open" to his viewpoint "even to the point of asking more questions, going deeper into the issue, about why I feel so strongly, and he understands where I stand," Mattis explained.
Mattis said he would work with other members of the Trump national security team "to carry these views forward."
Confronting RussiaRegarding Trump's favorable comments about Russian President Vladimir Putin, Mattis said his view of Putin is "that he has chosen to be both a strategic competitor" and an adversary in key areas. He expressed support for Trump's engagement with Russia "but I have very modest expectations about areas of cooperation with Mr. Putin."
Earlier in the hearing Mattis said he has foreseen an "increasing number of areas where we're going to have to confront Russia."
On the topic of resolving tensions between Israel and the Palestinian Authority, Mattis affirmed that the incoming administration would work to promote peace between the two adversaries.
It's "going to take time to build that kind of trust. And we should be a partner in trying to build that resolution between those people," he said. Mattis said he backed the two-state solution "if that brings peace to the middle east I'm eager to see it work."
An 'American Military Tradition'In his prepared remarks, Mattis called civilian control of the military "a fundamental tenet of the American military tradition."
"I recognize my potential civilian role differs in essence and in substance from my former role in uniform," he said. If Congress passes an exemption to the seven-year requirement, "I will provide strong civilian leadership of military plans and decisions," he added.
Mattis said his priorities as defense secretary would be "to strengthen military readiness, strengthen our alliances in league with our diplomatic partners and bring business reforms to the Department of Defense."
Moreover, he would "work to make sure our strategy and military calculus are employed to reinforce traditional tools of diplomacy, ensuring our President and our diplomats negotiate from a position of strength."
"It would be the highest honor, if I am confirmed, to lead those who volunteer to defend our country," he concluded.
Before Mattis can be confirmed as defense secretary, both houses of Congress will have to pass legislation that exempts Mattis from the legal barrier that prevents former members of the military from serving in that role until seven years have passed from the time they left military service. Mattis retired from the Marine Corps in 2013 after a 41-year career.
The legal restriction was enacted in the National Security Act of 1947 that created the Defense Department. A waiver has only been granted once, in 1950, to retired General George Marshall as he moved from his position as secretary of state to become the defense secretary under the Truman administration.
This afternoon the Armed Services Committees in both the House and Senate have voted in favor of a waiver for Mattis.
When he retired from the Marine Corps in 2013, Mattis was the commander of U.S. Central Command overseeing all American troops in the Middle East and with responsibility for the wars in Iraq and Afghanistan. That culminated a career with significant experience as a battlefield commander in Iraq during the Persian Gulf War and the 2003 invasion of Iraq. He was also in command of Marine forces in 2004 during both battles for Fallujah.
Along with Army General David Petraeus, Mattis co-authored the military's counterinsurgency strategy that helped turn the insurgent tide in Iraq in 2007.
Trump often refers to Mattis by the nickname of "Mad Dog" that describes his tenacity on and off the battlefield.
But he is also known as the "Warrior Monk," a nickname that refers to his singular focus on military history, tactics and strategy, traits along with his unmarried status.