President Biden has revised U.S. nuclear declaratory policy, which outlines the scenarios in which the U.S. would and would not use nuclear weapons. Henceforth, a fact sheet on the upcoming Nuclear Posture Review announced Mr. Biden’s policy that the “fundamental role” of our nuclear weapons is to deter nuclear attack, not, as previously declared, to deter both nuclear and nonnuclear attacks.

The new policy is not as good as the old, but it could have been worse. Mr. Biden has at least rejected the ill-advised calls from the far-left to declare the U.S. would never use nuclear weapons first, even in the face of the most destructive nonnuclear attacks. That policy would have deprived the president of an option that serves as a powerful deterrent to aggression. No wonder so many of our allies and senior officials publicly opposed taking the nuclear option completely off the table against nonnuclear threats.

But the bad news is that Mr. Biden’s revised declaratory policy still weakens deterrence at a time when it is needed most.

Prior to this announcement, the declaratory policy had been one of “calculated ambiguity.” It emphasized that the role of U.S. nuclear weapons is to deter both nuclear and nonnuclear attacks. Mr. Biden’s “fundamental role” policy walks this back, deemphasizing the role nuclear weapons play in deterring nonnuclear attacks.

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While we can expect a more comprehensive explanation once the full Nuclear Posture Review is released, Mr. Biden’s “fundamental role” policy appears very similar to that declared by former President Barack Obama in 2010. But 12 years ago, the threat environment was far more benign than it is today. Since then, Russia has invaded Ukraine (twice), Russian President Vladimir Putin has demonstrated an active chemical-weapons program with attacks on political opponents, and China has greatly expanded its military forces.

Indeed, the Trump administration strengthened Mr. Obama’s nuclear policy partly in recognition that our adversaries had greatly increased their lethal nonnuclear capabilities. Just look to Ukraine, where Russia is reportedly considering using chemical weapons to advance its aims.

To be clear, Mr. Biden’s policy does not significantly depart from the Trump policy in the way that a “no first use” or “sole purpose” policy would have. But given the increasingly dangerous threats posed by our more aggressive adversaries, it’s difficult to understand why Mr. Biden would bother doing anything at all to walk back a policy that wasn’t broken.

U.S. allies supported the Trump administration’s declaratory policy and have consistently messaged their opposition to making any changes. In fact, Deputy Under Secretary of Defense for Policy Sasha Baker testified in March: “I can’t say that I’ve found an ally who is urging us to reduce our nuclear deterrence or our declaratory policy in particular.”

If our allies did not advocate for this change in policy, then perhaps Mr. Biden is retreating to Obama-era nuclear policy to placate the left side of his party that favors nuclear disarmament. But altering policy that impacts nuclear deterrence—the number-one issue for U.S. national security—for political purposes would be nothing short of irresponsible.

Perhaps the administration believes that weakening U.S. declaratory policy will demonstrate to Russia and China that our intentions are more benign than those of the previous administration and will therefore help deescalate tensions.

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Such would be wishful thinking. More likely, this policy will suggest to adversaries and allies alike that the U.S. might be more reluctant to use nuclear weapons against major chemical, biological or conventional attacks. Our adversaries may interpret that message to mean they now have greater freedom to take more risks, so long as they keep the violence just below the nuclear threshold.

Moreover, weakening declaratory policy at a time when Mr. Putin is nuclear saber-rattling in Europe could signal that his threats are working, and that the U.S. is backing off. Mr. Biden’s decision to announce the reduced role of U.S. nuclear weapons as war wages on along NATO’s borders could also cause allies to question the administration’s assurance that it will live up to its extended deterrence commitments.

Unfortunately, there’s not much anyone can do to reverse this policy since the authority to use nuclear weapons lies solely in the president’s hands. But moving forward, support for a modern and flexible nuclear deterrent will remain more critical than ever. In the face of some of the greatest threats to national security, the U.S. must show strength.