We live in extremely dangerous times. It feels like August 1914, and the world’s leading actors are playing a game in which not everyone understands the rules.

I speak of the Ukrainian war. Kyiv’s recent successes on the battlefield are heartening. The aggressors are being pushed back. That is a good thing. But Ukrainian resistance is also pushing Vladimir Putin into a corner.

“In the event of a threat to the territorial integrity of our country and to defend Russia and our people,” Putin warned, “we will certainly make use of all weapon systems available to us. This is not a bluff.” He clearly meant nuclear weapons. If Russian forces are further routed, Putin could decide to use a tactical nuclear weapon—a so-called battlefield nuke.

In reality, there is no such thing as a battlefield nuclear weapon. The United States has around 200 tactical nukes with an explosive equivalency ranging from 0.3 to 170 kilotons of TNT. The Russians probably have at least 1,000 such “small” nukes. (By comparison, the Hiroshima bomb produced an explosion equivalent to about 15 kilotons of TNT.) Even the smallest tactical bomb could create serious devastation. The blast from even a one kiloton bomb would create a burn radius of 0.4 miles. Ostensibly, Putin’s generals could use such a weapon on an advancing column of Ukrainian tanks. But on the actual Ukrainian battlefield, there are very few targets large enough for even the smallest tactical nuke.

The real purpose of tactical nukes, however, is not to repel an enemy on the battlefield but to sow terror and change the psychological dynamics of the larger conflict. Indeed, nuclear weapons of any size have never been practical military weapons. They have only a political purpose. Back in November 1945—just three months after Hiroshima was destroyed—Robert Oppenheimer gave a speech in Philadelphia in which he calmly explained, “The pattern of the use of atomic weapons was set at Hiroshima.” The Hiroshima bomb, he pointed out, was used “against an essentially defeated enemy…. it is a weapon for aggressors, and the elements of surprise and terror are as intrinsic to it as are the fissionable nuclei.”

When asked recently on 60 Minutes what the US response would be, President Biden replied: “You think I would tell you if I knew exactly what it would be? Of course, I’m not going to tell you. It’ll be consequential. They’ll become more of a pariah in the world than they ever have been. And depending on the extent of what they do will determine what response would occur.”

Biden’s national security aides are no doubt advising him that Putin should be kept guessing on Washington’s response. But nuclear ambiguity has its dangers; it is time for nuclear clarity. Putin should be told the truth in no uncertain terms.

• Washington will not under any circumstances use nuclear weapons against Russia or against Russian forces in the Ukraine.
• If even one tactical nuclear weapon is used by Russia in the Ukraine, the West will redouble its supply of conventional weapons to the government in Kyiv.
• Russia will indeed become “more of a pariah.” Economic sanctions will be bolstered by a complete embargo.
• Putin will be indicted at the International Criminal Court for war crimes and a warrant for his arrest will be issued.

It is highly unlikely that Washington would contemplate using nuclear weapons against another nuclear power. That would be suicidal. On February 6, 2018, then–Secretary of Defense James Mattis stated, “I do not think there is any such thing as a tactical nuclear weapon. Any nuclear weapon used any time is a strategic game changer.”

Despite Biden’s pretense of strategic ambiguity, he clearly is not going to fight a nuclear war over Ukraine. And while it may be an official secret, our own generals know that nukes of any size are not viable military weapons. Since Americans are clearly not going to use nuclear weapons in Ukraine, why not say so?

Instead, we should be making it very clear to Putin that not even the use of tactical nukes will deter the Ukrainians from resisting the Russian invasion. Such clarity now might persuade Putin that he cannot expect to reap any political gains from the use of tactical nukes. It may seem counterintuitive, but we can deter Putin from using tactical nuclear weapons by proclaiming now that we will never use them ourselves on the Ukrainian battlefield.

By their willingness to fight, by their unlikely defiance of Putin, the Ukrainians have seized the moral high ground and deserve our support in terms of economic and military aid—but not American troops. To be sure, NATO expansion in the post–Cold War was a mistake, even a provocation. But NATO expansion neither justifies nor explains Putin’s war of aggression. This unilateral act of aggression by an unstable authoritarian actor has changed everything. Putin’s 2014 invasion and annexation of Crimea was also wrong, a violation of international law. Meaning that the Ukrainians are justified in their military efforts to take back the Crimea.

Nevertheless, we should take every opportunity to bring an end to this war through negotiations. So far, Putin has given no sign that he is willing to sit down and talk with the Ukrainians. But if talks did happen, if a Geneva peace conference were to be convened, America should also be willing to support a political compromise that ends the conflict.

Sadly, this war is likely to go on for some time. But whatever happens, we Americans should not countenance the use of a tactical nuclear weapon, however small, on the Ukrainian battlefield.