Defense Secretary Jim Mattis offered a justification for the Pentagon’s amped-up nuclear weapons policy, saying that at least one of two new nuclear arms the U.S. military wants to introduce could be used as a bargaining chip with the Russians. 

Mattis, who testified Tuesday before the House Armed Services Committee, entered the Pentagon last year with questions about the military’s plans to overhaul the nuclear arsenal. He wondered whether the United States needed to retain its Cold War-era intercontinental ballistic missile silos or approve a new air-launched cruise missile. 

But after nearly a year of analysis and consultations with military commanders and outside experts, Mattis endorsed the modernization in full — and even agreed to augment the plans. 

The policy the Pentagon rolled out last week calls for the reintroduction of submarine-launched nuclear cruise missiles and new “low-yield” nuclear warheads to put on submarine-launched ballistic missiles. Despite being described as low-yield, such weapons could cause roughly as much damage as the bombings of Hiroshima and Nagasaki in Japan.

Mattis said Tuesday that his decision to reintroduce the submarine-launched cruise missiles — SLCMs or “slick-ems” in military parlance — comes in response to Russia’s violations of the Intermediate Range Nuclear Forces (INF) Treaty, which Moscow and Washington signed in 1987 to eliminate medium-range missiles. He said he hoped U.S. diplomats could persuade Russia to come back into compliance.

“I want to make sure that our negotiators have something to negotiate with,” Mattis said. “We want Russia back in compliance. We do not want to forgo the INF. At the same time, we have options if Russia continues to go down this path.”

Congress mandated in its latest annual defense policy bill that the Pentagon begin drawing up plans for a U.S. missile banned under the INF Treaty, which the military could deploy in the event that the pact falls apart. The treaty bans production and deployment of such missiles but not research and development. The Pentagon hopes that R&D program will also put pressure on Russia.

The United States hasn’t publicized specifics about Russia’s treaty violation, but outside experts think the Pentagon is referring to Russia’s alleged deployment of an SSC-8 ground-launched cruise missile. Russia, in turn, has accused the United States of violating the treaty with its Aegis missile-defense installations in Europe, an allegation U.S. officials have denied.  

Mattis said Tuesday that the United States should seek more communication with Russia and China that goes beyond operational matters to address topics such as arms control. The defense secretary called it more “philosophical engagement” with Moscow and Beijing.

He also said the Pentagon’s plans to introduce low-yield warheads on submarine-launched ballistic missiles responds to scenarios Russia has run in which Moscow would seize part of a U.S. ally state and use a small nuclear weapon to deter the United States from intervening. The United States would then be forced to choose between responding with much larger nuclear weapons or conventional arms, according to the Pentagon, which wants an option it says the military now lacks.

Disarmament advocates say the introduction of such weapons lowers the threshold for nuclear war, making it easier to justify their use in a “limited capacity.” They also warn that Mattis’s decision to approve new weapon-delivery systems more broadly risks catapulting the United States into an arms race. 

Bruce Blair, the co-founder of Global Zero, which promotes the elimination of nuclear weapons, said in a statement in response to the new policy last week that the United States withdrew low-yield nuclear arms deployed during the Cold War “out of an abundance of caution about their security, risks of accidents and unauthorized use, and potential to spark a nuclear exchange.”

Blair warned that augmenting the U.S. arsenal with nuclear weapons that seem more “usable” raises the likelihood of nuclear war.

Mattis said that the new low-yield weapon would in fact raise the threshold for nuclear war by deterring Russia from engaging in such an attack. Disarmament advocates reject that reasoning.  

“I don’t think there’s any such thing as a tactical nuclear weapon,” Mattis said, referring to the small “battlefield” nuclear weapons that Russia possesses in vast quantities. “Any nuclear weapon used at any time is a strategic game changer.”

The United States already has low-yield nuclear weapons in its arsenal. The best known one, the B-61 gravity bomb, would require a U.S. bomber pilot to penetrate enemy air defenses and drop it on a target, Mattis said. He said the advancement of air defenses in recent decades requires a submarine option.  

“We are certainly working on air-defense-penetration capability,” Mattis said. “But again, we have to deal with where we’re at today.”  

Mattis appeared alongside Gen. Paul J. Selva, vice chairman of the Joint Chiefs of Staff. Both said the modernization program was necessary to maintain the U.S. deterrent. The Congressional Budget Office has estimated it will cost approximately $1.2 trillion to operate, modernize and sustain the United States’ nuclear forces over 30 years. 

It remains to be seen whether and when Congress will appropriate funds for the capabilities the Pentagon outlined in last week’s nuclear weapons policy.

Rep. Adam Smith (Wash.), the Armed Services Committee’s highest-ranking Democrat, said he understood the need for the nuclear modernization but wondered how the United States would pay for it, given that Congress decided to “give away $2 trillion” of government money in the new tax law.

“How do we make this fit? How does this work?” Smith said. “I worry greatly about how this strategy is going to be implemented in the face of our debt and our deficits.”