It’s a key question in the debate over the Trump administration’s Nuclear Posture Review:

Does adding a couple of low-yield weapons to each of America’s ballistic missile submarines make the U.S. safer, or move the U.S. closer to nuclear war?

Defense Secretary Jim Mattis believes a low-yield option that doesn’t rely on an airplane having to penetrate enemy air defenses increases deterrence, and therefore lowers the risk of miscalculation, especially by the Russians.

It’s a perfectly logical argument, but it’s one that arms reduction advocates and skeptical members of Congress have a hard time accepting.

In his testimony before the House Armed Services Committee Tuesday, Mattis tried repeatedly to make the case that the whole point of having a handful of low-yield missile warheads was not for the U.S. to use them, but to make sure America’s enemies doesn’t use theirs.

Mattis invoked the name of former Secretary of State Henry Kissinger, who testified before Congress last month that while the U.S. may have no offensive need for smaller tactical nuclear weapons, it doesn’t want to be in a position where its only response is an all-out nuclear war.

“If the technology develops in such a way that other major countries possess them, we should think carefully before we put ourselves into a position where our only response is an all-out nuclear strike,” Kissinger testified before the Senate Armed Services committee Jan. 25.

Or as Mattis put it, “We don’t want someone else to miscalculate that because they are going to use a low-yield weapon, that somehow we would confront what Dr. Kissinger calls ‘surrender or suicide.’ We do not want even an inch of daylight to appear in how we look at the nuclear deterrent.”

Mattis made clear that the low-yield option was in direct response to Russia’s doctrine of “escalate-to-deescalate.”

“What it does is it makes very clear that we have a deterrent if the Russians choose to carry out what some of their doctrine people, their political leaders have promoted, which would be to employ a low yield nuclear weapon in a conventional fight in order to escalate-to-deescalate, in other words to escalate to victory, and then deescalate,” Mattis said. “We want to make certain they recognize that we can respond in kind, and that we don’t have to go with the high yield weapon.”

The U.S. currently has a low-yield version of the B-61 nuclear gravity bomb, which can only dropped from an airplane, such as a stealthy F-35.

The new plan is to augment that capability by retrofitting about two dozen existing W-76 warheads that are currently deployed on submarine-launched D-5 Trident missiles.

The current high-yield warhead has a two-stage design in which a small fission explosion triggers a much more powerful fusion reaction.

On some of the warheads, the fusion component would be removed, leaving only the smaller fission device, resulting in a lower yield that can be delivered by a intercontinental ballistic missile launched from a submarine.