The U.S. military has put low-yield nuclear warheads into operation on submarines, citing the need to deter a limited nuclear attack by Russia with similarly small warheads in a scenario that worries Pentagon planners.

In a statement released Tuesday, Undersecretary of Defense for Policy John C. Rood confirmed that the W76-2 low-yield warhead had been fielded on submarine-launched ballistic missiles. The introduction of the warhead on Ohio-class nuclear submarines marks the first time the U.S. military has armed underwater vessels with warheads that can explode at such small yields since the George H.W. Bush administration.

The move comes over the objections of top Democrats and antinuclear advocates who have called it dangerous.

Rood said the W76-2 strengthens deterrence against adversaries and gives the United States a low-yield option that is more survivable in the event of a nuclear war. The U.S. military already possesses a low-yield option in the B61 gravity bomb, but that warhead and its variants can be launched only from aircraft, which the Pentagon believes could be stymied by sophisticated Russian air defenses.

The introduction of the W76-2 on American submarines, planned since the beginning of the Trump administration, “demonstrates to potential adversaries that there is no advantage to limited nuclear employment because the United States can credibly and decisively respond to any threat scenario,” Rood said.

In a review of nuclear policy overseen by then-Defense Secretary Jim Mattis, the Pentagon determined that there was a gap in U.S. nuclear capabilities vis-a-vis Russia.

Officials argued that Russia could employ one of its many small nuclear weapons in a limited attack against an American ally, potentially forcing the United States to choose between responding with a high-yield strategic nuclear warhead, all but guaranteeing full-scale nuclear war, or returning fire with a conventional weapon, risking embarrassment or defeat. The Pentagon refers to this strategy, which it has attributed to Russia, as “escalate to de-escalate” or “escalate to win.”

Top Russian officials have denied such a strategy exists. They have said Russian nuclear doctrine calls for the use of nuclear weapons only when one is used first by an adversary against Russia or its allies, or when the use of conventional weapons against Russia puts the state at risk. Russian President Vladi­mir Putin has said Russia doesn’t envision conducting a preemptive strike with nuclear weapons.

Current and former U.S. officials, however, argue that Russian writings on nuclear doctrine and exercises demonstrate the existence of an “escalate to de-escalate” strategy, which could back the United States into a corner if the U.S. military lacks weaponry to respond in a like-for-like manner.

“It’s necessary to have this capability to close a gap in the credibility of our deterrence, because you have to put yourself in the mind not of some nuclear disarmament advocate but of a Russian general or the Kremlin,” said Tim Morrison, a senior fellow at the Hudson Institute and former senior director for weapons of mass destruction on the National Security Council under President Trump.

“If they think the use of a very low-yield nuclear weapon by Russia would not credibly be responded to by the United States with a higher-yield weapon, because it would be disproportionate, you have created a gap in your deterrence that the administration is trying to close,” Morrison said.

The U.S. military created the new warhead now fielded on Ohio-class submarines with Trident ballistic missiles by modifying a larger-yield W76-1 warhead.

Official U.S. nuclear-warhead yields remain classified, but experts estimate that the new W76-2 would explode with a yield of about 6.5 kilotons, whereas the full-size W76-1 explodes with a yield of roughly 90 kilotons. By comparison, the warheads the U.S. military used on Hiroshima and Nagasaki, Japan, in 1945 exploded with about 15 and 20 kilotons of force, respectively.

Critics have said the introduction of the low-yield warhead on submarines could potentially confuse an adversary, who in the event of a launch might think it’s a high-yield attack.

“Mixing these indistinguishable low-yield weapons alongside high-yield warheads creates ambiguity and could lead to a massive escalation,” said Andy Weber, former assistant secretary of defense for nuclear, chemical and biological defense programs during the Obama administration.

In 2018, then-California Gov. Jerry Brown (D) and prominent former U.S. officials wrote to the Senate majority leader describing the W76-2 as a gateway to nuclear catastrophe.

The letter — signed by 32 people, including former secretary of state George Shultz and former defense secretary William Perry — called the rationale for the low-yield warheads a “false narrative.”

The officials said the introduction of the warheads was based on a “mistaken and dangerous belief” that it would be possible to prevent a limited nuclear exchange between Russia and the United States from escalating into an uncontrollable nuclear war. They also said the Pentagon already has low-yield options, making the addition unnecessary.

“Ultimately, the greatest concern about the proposed low-yield Trident warhead is that the president might feel less restrained about using it in a crisis,” the letter said. “When it comes to using a nuclear weapon, restraint is a good thing.”

At the time, Mattis issued a rebuttal, calling the move a modest adjustment necessary because of developments in Russian doctrine, exercises and capabilities.

“Let me be clear, any decision to employ nuclear weapons would be the most difficult decision a President has to make,” Mattis wrote. “This Administration, like the ones before it, has said that nuclear weapons would be employed only in extreme circumstances to protect our vital interests and those of our allies and partners.”