WASHINGTON — Secretary of Defense James Mattis has asserted on Capitol Hill that the Pentagon needs to invest in low-yield nuclear weapons to counter a Russian strategy of “escalate to de-escalate” — firing low-yield nuclear weapons with the belief that the United States would not retaliate because it would have to use full-scale nuclear weapons.

Experts don’t agree on whether the low-yield option Mattis suggests — which the Defense Department took off the table in the 1990s — is the correct deterrence approach, and more importantly, they don’t agree that “escalate to de-escalate” is actually the Russian doctrine. Many experts argue that this doctrine does not line up with what they know of Russia’s strategy.

Before breaking down each side, let’s define our terms.

Low-yield nuclear weapon

When defense officials talk about low-yield nuclear weapons today, they mean a kind of smaller-scale nuclear device that can be launched from a submarine. The administration wants to incorporate low-yield weapons on both submarine-launched ballistic missiles and sea-launched cruise missiles.

The current U.S. arsenal of low-yield weapons is delivered from planes, but Russia’s anti-aircraft defenses can stop them.

“Smaller-scale” is a relative term. A 20-kiloton “low-yield” device causes destruction on par with the Hiroshima and Nagasaki bombings, each of which killed tens of thousands in the initial blast alone.

Escalate to de-escalate

While there’s not agreement that this nuclear doctrine is Russia’s strategy, the thinking goes like this: Because Russia has a large arsenal of low-yield “tactical” nuclear weapons, and the United States is limited to the air-launched variety that Russia can shoot down, the United States wouldn’t retaliate because its only option would be to launch full-scale nuclear weapons. The next step would be Russian full-scale nuclear retaliation — mutually assured destruction.

Here’s an example of what that might look like: What if Russia made a quick land grab in the Baltics to make up for what it sees as the loss of its security perimeter. While Russia has a regionally dominant conventional military force, it is underpowered compared to NATO forces. Using the low-yield nuclear strategy would, under this doctrine, halt a conventional response as well as a nuclear response because NATO members would be hesitant to jump too fast up the escalation ladder.

Why it might work

“I am willing to speculate that Russia understands all too well the psychological inhibitions that the United States has placed upon itself, having been the only country to use nuclear weapons in war,” said I.D. Hendrix, senior fellow and director of the Defense Strategies and Assessments Program at the Center for New American Security. “It gives them a lot more leeway when they start ratcheting up the tensions.”

Mattis, speaking at a House Armed Services Committee oversight hearing on Feb. 6, said low-yield nuclear weapons prevent “miscalculations” by the Russians that this leeway could cause.

But even without new low-yield weapons, Hendrix said in an interview he doesn’t see any situation in which the United States would not issue an immediate response.

“When all you’ve got is a hammer, everything looks like a nail,” Hendrix said. “The idea of reintroducing or doing a modernization effort that actually brings some tactical nuclear weapons back into the inventory puts more tools back into the tool box.”

Tighter control over the escalation ladder could also bring Russia to the bargaining table, where diplomats could have discussions about deterrence, Hendrix said, adding, “What [Mattis] is doing is stimulating a meeting of the minds.”

Mattis makes a point to emphasize his belief that rather than lowering the threshold to nuclear war, low-yield weapons give negotiators a bargaining chip. He said they would only be used “in the most extreme circumstances” — he even prefers not to use the term “nuclear bombs” and insists on specifically calling them “nuclear deterrents.”

“The deterrence effort stays primary,” Mattis said. “It is not to in any way lower the threshold to use nuclear weapons.”

Why it might not

By keeping the attention of the United States fixed on nuclear threats, Russia gains the space to focus on buttressing its regional military dominance without interference from NATO, said Kingston Reif, director of disarmament and threat reduction policy at the Arms Control Association.

“Russia does have more non-strategic or tactical nuclear weapons than the United States,” Reif said. “By attempting to mimic Russia by developing more low-yield options I think we play into Moscow’s hands to some extent because Russia can match NATO in the nuclear sphere” but comes up short in the conventional weapons arena.

Reif said the U.S. acquisition of low-yield nuclear bombs could have the “perverse effect of potentially convincing Russia that it could get away with limited nuclear use without putting its survival at risk.”

He also believes that, in the event of the firing of a low-yield weapon, the idea that the nuclear escalation ladder can be tightly controlled, as Hendrix and Mattis have suggested, is flawed.

“The fog of war is thick but the fog of nuclear war would be even thicker,” Reif said. “Once the nuclear shooting starts, a nuke is going to be a nuke. It’s an incredibly risky bet to think…that cooler heads would prevail.”

The bottom line

It is not clear this doctrine is Russia’s current policy for using nuclear weapons. There is some evidence indicating that Russia is working to raise the threshold for nuclear war, not lower it. Experts point to Russia’s investments in conventional weapons and cyber warfare as demonstrative of the fact that it is looking for alternatives to tactical use of nuclear devices.

“The claim that Russia has such a doctrine…is hotly disputed,” Reif said. “It’s fair to say that Russia relies more on nuclear weapons for security than the United States due to Moscow’s overall conventional inferiority, but Russia’s official nuclear doctrine does not support the claim.”

Russia may prefer nuclear saber rattling to diplomacy because it helps them “identify and exploit ways to keep the West off-balance, including nuclear brandishing,” Olga Oliker, senior adviser and director of the Russia and Eurasia Program at the Center for Strategic and International Studies, wrote in a report.

“Officials are careful to avoid directly contradicting doctrine, but happy to make vague and ominous pronouncements,” Oliker wrote. “This is unnerving, but it falls short of a convincing case of a coherent strategy of ‘escalating to de-escalate.'”

The original theory behind deterrence strategy is that nuclear war would be so destructive as to be unwinnable. Destruction would be mutually assured. Regardless of whether Russia believes in “escalate to de-escalate,” the existence of low-yield nuclear weapons on either side could unhinge this cornerstone of policy.

“Any nuclear weapon used any time is a strategic game-changer,” Mattis said.

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