Defense Secretary Jim Mattis has rescinded doubts over the U.S. nuclear arsenal after a months-long review. (Jacquelyn Martin/AP)

When retired Marine Corps Gen. Jim Mattis became defense secretary last year, he arrived at the Pentagon with reservations about the U.S. nuclear arsenal.

He had doubts about an ­air-launched nuclear cruise missile under development at the Pentagon and questioned whether the United States should have intercontinental ballistic missile (ICBM) silos at all.

But in the year since then, Mattis has changed his tune. The nuclear weapons policy his team rolled out at the Pentagon last week offered full-throated support for the military’s current and planned nuclear capabilities, including the new cruise missile and the ICBM fleet he once questioned.

The strategy marks a resounding win for backers of the U.S. nuclear enterprise and a setback for disarmament advocates. Mattis is scheduled to testify on the matter Tuesday on Capitol Hill. 

The policy reaffirms a full modernization of the U.S. nuclear force approved by President Barack Obama, which replaces the military’s nuclear bombers, submarines and ICBMs at an estimated cost of $1.2 trillion over 30 years. It calls for two types of nuclear weapons not currently in the arsenal. 

Secretary of Defense Jim Mattis gave a pointed opening statement during the Senate Armed Services Committee's hearing on the national defense strategy. Mattis said that stopgap measures like continuing resolutions harm national security and the readiness of the armed forces. (Senate Armed Services Committee)

Mattis’s evolution in thinking offers insight into the decision-making process of a defense secretary known for his intellectual rigor. His evaluation of the military’s nuclear weapons capabilities also serves as a counterpoint in an administration whose policymaking process has often appeared hurried and impulsive. 

Thomas Karako, a specialist on nuclear issues at the Center for Strategic and International Studies in Washington, said many people — including Obama — enter government sympathetic to disarmament but moderate their ambitions when confronted with threat intelligence. 

“I would point out a pattern of being mugged by reality,” said Karako, who wasn’t involved in the evaluation.

In the preface to the policy, Mattis appeared to confirm that sentiment. “We must look reality in the eye and see the world as it is, not as we wish it to be,” he wrote. 

 Over more than four decades in the Marines, Mattis had little direct involvement with the nuclear arsenal, which is typically the domain of the Air Force and Navy. 

He delved into the issue at Stanford University’s Hoover Institution after his retirement. There, he exchanged views with disarmament advocates including former secretary of state George Shultz and his onetime boss at the Pentagon, former defense secretary William J. Perry. 

Perry has argued that the United States should retire its land-based ICBM force. The U.S. bombers and submarines that carry nuclear warheads offer sufficient deterrence, he has said, and the ICBMs, which sit in fields ready to launch at any moment, could accidentally trigger nuclear war. 

Perry has also said adversaries could confuse the cruise missile the Pentagon is developing for a conventional missile. 

Mattis at times has nearly echoed such views. In 2015 testimony to the Senate Armed Services Committee, he said it was time for the Pentagon to consider  retiring the land-based ICBM force. During his 2017 confirmation, Mattis said he needed to “look at” the cruise missile. 

President Trump commissioned a review of nuclear weapons policy a week after taking office. For Mattis, it became an opportunity to reeducate himself about the U.S. nuclear force, this time with full access to the latest intelligence reports and the U.S. military. 

He visited the ICBM force and B-52 bomber fleet in North Dakota. He stopped by a nuclear submarine base in Washington state. He spoke to the generals at Strategic Command in Nebraska.

“I can’t think of a secretary who so quickly visited all the nuclear forces,” said Rob Soofer, deputy assistant secretary of defense for nuclear and missile defense policy.

All the while, Mattis read widely about nuclear weapons, according to people with knowledge of the process who spoke on the condition of anonymity because of the review’s sensitivity. One person said Mattis referenced works including Yale Professor Paul Bracken’s “The Second Nuclear Age,” which argues for a post-Cold War approach to arms control. 

Mattis has said he pressed retired officers, scientists and professors for their views. In addition, he convened a council of retired admirals, generals and specialists known as “the graybeards” to engage in what Soofer said Mattis calls a “skilled dialectic,” or an exchange of divergent views.

“I’ve got the smartest people I can find,” Mattis told nuclear submarine officers in Washington. “And it’s Republican and Democrat. It’s men and women. It’s old people. It’s young physicists. It’s people who studied history. There are people who know what they’re doing.”

Convening people who disagree is a well-known Mattis tactic. When he led Joint Forces Command, he set up an advisory board that included both Hillary Clinton and Newt Gingrich. 

By the time he visited Naval Base Kitsap last August, Mattis appeared to have all but decided. 

Ultimately, Mattis took those and other recommendations to the White House. To have done otherwise would have forced him to confront tremendous pressure from Congress, the military and the White House, all of which backed the new policy.

According to people who spoke with him, Mattis committed to the cruise missile in part because he believes it could penetrate air defense systems adversaries are likely to develop. He decided to retain the ICBM force because he thinks the hundreds of silos across the heartland serve as an important deterrent for Russia and other potential adversaries, given how difficult it would be for an enemy to disable them all in one shot.  

Some disarmament advocates view the results of the review with dashed hope.   

“I found it disappointing that this document includes misguided framing and ideas that have long been on the wish list of conservative nuclear policy analysts and government officials,” said Kingston Reif, director of disarmament and threat reduction policy at the Arms Control Association.

Still, Reif held out hope. He said Mattis’s signed preface to the new policy appeared to take a more restrained tone than the rest of the document. Reif added, “It does remain to be seen how vociferously and strongly Mattis will go to bat for some of the proposals.”