(Sarah Parnass/The Washington Post)

Retired Marine Gen. James Mattis, Donald Trump’s pick to be the next U.S. defense secretary, placed Russia first among principal threats facing the United States, arguing for greater American support for European allies to counter what he said were Moscow’s attempts to shatter the North Atlantic security alliance.

Mattis, a respected combat leader who made his name in the wars following the Sept. 11, 2001, attacks, cautioned that sustained cuts to military budgets and personnel meant the U.S. military is no longer strong enough to easily outmatch Russia and other adversaries.

Mattis’s remarks during his confirmation hearing Thursday provide some of the first hints about how the Trump administration, which has not put forward comprehensive national security plans, may alter the posture of the world’s most advanced military at a time of institutional strain and uncertainty about the future.

His comments also signal a possible divergence in viewpoint with the president-elect, who has questioned long-standing security commitments and voiced his willingness to partner with Russian President Vladi­mir Putin. Trump said at a news conference Wednesday that “Russia can help us fight” the Islamic State, but he also noted: “I don’t know that I’m gonna get along with Vladimir Putin. I hope I do. But there’s a good chance I won’t.”

Mattis’s seeming differences with Trump and those with the president-elect’s pick to be secretary of state, Rex Tillerson, who unlike Mattis has suggested that he might support renegotiating President Obama’s nuclear deal with Iran, could lead to a fractious approach to foreign policy and interagency feuding in the next administration.

Mattis spoke as Rep. Mike Pompeo (R-Kan.), whom Trump has tapped to lead the CIA, pushed back on Trump’s support for waterboarding during the campaign and said he would reject orders to torture detainees.

The conflicting statements from the men who will be Trump’s most senior advisers increase uncertainty about what actions the new administration will take as military leaders continue to battle the Islamic State and grapple with growing challenges from China and North Korea.

Several hours after Mattis’s testimony before the Senate Armed Services Committee, the Senate voted 81 to 17 to waive a measure requiring defense secretaries to have been out of military service for seven years. Mattis retired in 2013.

Later in the day, the House Armed Services Committee approved a similar measure along party lines, 34 to 28. The full House must also vote on that measure.

Although some of the president-elect’s other Cabinet picks have come under intense questioning in their own confirmation hearings, Mattis encountered virtually no challenges from lawmakers to his suitability for the top Pentagon job.

The 66-year-old veteran, known for his use of the call sign “Chaos” during overseas deployments, has earned a reputation as a scholarly, plain-spoken officer with an impressive combat record. His blunt style has brought controversy at times, as have his hawkish views on confronting threats in the Middle East.

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Mattis was named the head of U.S. Central Command in 2010, but he left in 2013 amid disagreement with the Obama White House over the general’s desire to intensify the military response to Iranian activities throughout the region.

Iran remains “the primary source of turmoil” in the Middle East, Mattis told lawmakers on Thursday, with its support for regional militant cells, its ballistic missile capability, its maritime provocations and cyber initiatives.

Although Mattis’s hawkish views on the danger from Iran appear to coincide with Trump’s, he broke with the president-elect by voicing support for leaving the nuclear deal with Tehran intact.

“I think it is in an imperfect arms control agreement — it’s not a friendship treaty,” he said. “But when America gives her word, we have to live up to it and work with our allies.”

The questioning was notable for its scant discussion of the wars in Afghanistan, where 8,400 U.S. troops are stationed; in Iraq, where about 6,000 Americans are supporting a punishing Iraqi offensive against the Islamic State; and in Syria, where a small Special Operations force hopes to help local forces drive the militants from their stronghold of Raqqa.

Mattis did say that the U.S. strategy for Raqqa “needs to be reviewed and perhaps energized on a more aggressive timeline.”

Speaking to lawmakers about Russian activity beyond its borders, Mattis gave a full-throated defense of NATO and said he supports the Pentagon’s European Reassurance Initiative, which has added military power in eastern Europe in response to concerns about Russian pressure on the Baltics. On Thursday, Putin’s spokesman criticized the United States’ decision to begin a major deployment of troops and heavy equipment near Russian borders in eastern Europe.

“Since Yalta, we have a long list of times we’ve tried to engage positively with Russia. We have a relatively short list of successes in that regard,” Mattis said, referring to the Yalta conference at the close of World War II. “The most important thing is that we recognize the reality of what we deal with Mr. Putin and we recognize that he is trying to break the North Atlantic alliance, and that we take the steps . . . to defend ourselves where we must.”

As he did with Iran, Mattis, despite his harsh rhetoric, provided few specific ideas for using military means to push back against Russia.

It’s unclear whether Mattis’s views will spark a confrontation with the Trump White House. Trump’s apparent interest in partnering with Moscow against the Islamic State in Syria, for example, may renew resistance that such ideas have provoked among military leaders in the past.

Mattis repeatedly spoke up for traditional alliances, including those with NATO, South Korea and Japan, that Trump has questioned. The president-elect has expressed skepticism about U.S. security commitments unless those partners increase outlays for their own defense.

“We must also embrace our international alliances and security partnerships. History is clear: Nations with strong allies thrive and those without them wither,” Mattis said.

The nominee said Trump has shown himself to be open and inquisitive while discussing NATO with him. Asked how he, a retired four-star general, would negotiate his relationship with Trump’s pick for national security adviser, retired three-star general Michael Flynn, Mattis said that debate of policy issues “isn’t always tidy” but he didn’t expect problems.

Asked about the possibility that Trump’s Cabinet nominees may differ with him on Russia, incoming press secretary Sean Spicer said that the president-elect was “not asking for clones” in selecting his senior advisers. Still, he said, “at the end of the day, each one of them is going to pursue a Trump agenda and a Trump vision.”

Democratic lawmakers expressed hope that Mattis, forged by the military establishment and boasting experience across administrations of both parties, would act as a restraint on some of Trump’s impulses.

Repeatedly, the retired general referred to the need to improve military readiness, blaming years of budget cuts for an erosion to American technology and manpower. Trump has promised to increase defense spending.

Mattis, who comes from a service that has led pushback to efforts to integrate women into combat positions, also signaled openness to allowing female service members to serve in all jobs as long as they meet established requirements.

Sen. Kirsten Gillibrand (D-N.Y.), a strong supporter of gender integration, pointed to comments Mattis made after his retirement questioning the wisdom of placing women serving in all combat roles. Mattis responded that he has no plans to oppose women in any roles within the military. He said that he had hundreds of women serving among his 23,000 Marines during the invasion of Iraq in 2003.

“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, but I’m not coming in looking for problems,” he said. “I’m looking for a way to get the department so it’s at the most lethal stance.”

Anne Gearan, Karoun Demirjian, Joby Warrick, Kelsey Snell and Ed O’Keefe contributed to this report.