WASHINGTON — Retired Gen. James Mattis, poised to become the first career military officer to lead the Pentagon since the 1950s, said Thursday he believes Russian President Vladimir Putin is trying to “break” the NATO alliance that has anchored American and European security for more than half a century.

Under questioning at his Senate confirmation hearing, Mattis portrayed Russia as a strategic adversary. He said the history of U.S.-Russian relations is short on successful efforts at lasting cooperation, something President-elect Donald Trump has pledged to pursue.

Of Putin, said Mattis, a former NATO military leader: “He is trying to break the North Atlantic alliance.”

And he said Trump was “open” to his views on Russia and has asked him about how he sees the relationship.

Mattis said he supports the Obama administration’s moves to reassure European allies after Moscow’s annexation of Ukraine’s Crimea region and military activity in eastern Ukraine. While the U.S. should remain open to working with Russia, Mattis said, the prospects for cooperation are narrowing.

The context was clear. As Mattis spoke, Trump’s choice to run the CIA was testifying before another Senate panel and was sure to be questioned at length about the U.S. intelligence agencies’ allegations that Moscow interfered in the presidential election. Ties between the former Cold War foes also have been strained by Syria’s civil war.

More broadly, Mattis said the world order was under “the biggest attack since World War II,” blaming Russia, China and international terrorist organizations for destabilizing trends and actions.

Asked by Sen. John McCain, the Armed Services Committee’s Republican chairman, whether the U.S. military was fully ready to confront these challenges, Mattis replied: “No, sir.”

On cyberattacks, Mattis noted that wars often are started by miscalculation. He said the U.S. needs to set clear boundaries so that adversaries know what the U.S. will not tolerate.

A former top commander of U.S. forces in the Middle East, Mattis said he believes Washington must “restore a better relationship” with Israel and Arab partners. Still, he declined to embrace the common Republican assertion that President Barack Obama has allowed Israel’s military edge over its regional foes to erode.

In prepared testimony, Mattis said he understands his role as the Defense Department’s civilian leader would be different “in essence and in substance” from his four decades in uniform.

“The esprit-de-corps of our military, its can-do spirit and its obedience to civilian leadership reduces the inclination and power of the military to criticize or oppose the policy it is ultimately ordered to implement,” Mattis said. He called civilian control “a fundamental tenet of the American military tradition.”

Mattis, 66, retired in 2013 after serving as commander of U.S. Central Command in charge of all U.S. forces in the Middle East.

He is known for strong views on Iran. Mattis sees Tehran as a menace in the Middle East, and would work for a president who has pledged to toughen U.S. policy toward Iran. That could have broad implications as the incoming administration weighs modifying the 2015 Iran nuclear deal and reconfiguring American posture in the Middle East after complaints from U.S. allies that Obama yielded too much ground to Tehran.

The last career military officer to serve as defense secretary was George Marshall in 1950-51.

In his opening remarks to senators, Mattis expressed unqualified support for traditional U.S. international alliances. In contrast, Trump insisted during the presidential campaign that U.S. treaty allies and security partners should pay more for their own defense and for hosting American forces on their soil.

“History is clear: Nations with strong allies thrive and those without them wither,” Mattis said.

Before Mattis can join the Cabinet, Congress must approve a one-time exception to a law requiring a military officer to be out of uniform for at least seven years before leading the Pentagon. Even some of Trump’s strongest critics say Mattis merits the exception.