Gen Mattis also gave a strong endorsement of the North Atlantic Treaty Organisation alliance that was formed to counter Soviet aggression during the Cold War, which may allay concerns about Mr Trump's comment during his presidential campaign that the United States may not keep its commitment to defend North Atlantic Treaty Organisation allies unless they spend more on their military budgets.
Thornberry said the transition team told him they were concerned about adding extra public settings for questions that could complicate the confirmation process, as well as the fact that Mattis would be testifying on civilian control specifically in the House, and not the Senate, despite his Thursday hearing in the Senate.
"I'm coming in with the understanding that I lead the Department of Defense and if someone brings me a problem then I'll look at it", Mattis said.
Retired Marine Corps General James Mattis said Russia, China and militants were presenting the biggest challenge to the US-led world order since World War Two, and called for Congress to lift spending caps undermining military readiness.
India is the worlds largest democracy and "our relationship with it is of utmost importance", he said.
Ben Carson, Mr Trump's nominee for housing secretary, is also being questioned by senators.
Former Georgia Senator Sam Nunn pushed for a statutory waiver while touting Mattis' qualifications for the position when he introduced the General Thursday morning. For some, the waiver represents a lack of concern for the principle of civilian rule over the military. Claire McCaskill, a Missouri Democrat, and Kirsten Gillibrand, a New York Democrat, Mattis said he would support the current policy on women in combat roles and gays serving openly.More news: 5 things Andy Murray Can Aim For In 2017
In addition to his comments on Russia, Mattis broke with the president-elect when asked broadly about the intelligence community, saying that he had worked closely with them throughout his time in the military and that he has "a very, very high degree of confidence in our intelligence community".
Unceremoniously forced out of the Pentagon by the Obama administration over his opposition to the Iran nuclear deal, Gen. Mattis was named as Mr. Trump's pick for defense secretary in December.
During his confirmation hearing, Mattis said he understood the difference between military and civilian responsibilities in government and promised to "provide strong civilian leadership of military plans and decisions".
Due to enter the White House in eight days, Trump has acknowledged that Russian Federation likely hacked the Democratic National Committee and emails of top Democrats during the 2016 presidential election campaign, a conclusion reached by USA spy agencies. He retired in 2013, well short of the seven-year requirement.
Of great concern is Mattis' ideas on the service of LGBT members.
Mattis joins Trump picks who have taken a strong stance on Russian Federation during confirmation hearings this week. Army Gen. George C. Marshall, an active-duty five-star general, was defense secretary for about a year beginning in 1950 after he was selected by President Truman as the military struggled in the early days of the Korean War.
"I think it is an imperfect arms control agreement", he said.