As he sent Russian soldiers to invade Ukraine, Putin ordered his nuclear arsenal to “special combat readiness” and threatened “consequences as you have never experienced in your history.” After seven decades in which no nuclear weapon has been used in war, many today assume that a “nuclear taboo” makes any deliberate use of nuclear weapons unthinkable. We suggest they think again.

A person who did not hesitate to bomb one of his nation’s own cities into rubble could certainly contemplate using low-yield nuclear weapons to destroy a Ukrainian city. Exploring that path, he might even take a page from the U.S. playbook in ending World War II. Putin could consider delivering a low-yield nuclear weapon to destroy one of Ukraine’s small cities, call on Zelenskyy to surrender—and threaten that if he did not, invite him to watch what a Ukrainian Nagasaki looks like.

As the United States and its partners contemplate the road ahead, we urge them to remember the lesson that President John F. Kennedy offered successors as the main takeaway from the Cuban Missile Crisis. In his words: “Above all, while defending our vital interests, nuclear powers must avert those confrontations which bring an adversary to a choice of either a humiliating retreat or a nuclear war.”

In conclusion, to return to the fundamental truths about war with which we began, our answers to these core questions remind us of two more truths. Once begun, wars take on a life and momentum of their own. And finally: war is hell.

Graham T. Allison is the Douglas Dillon Professor of Government at the Harvard Kennedy School. He is the former director of Harvard’s Belfer Center and the author of Destined for War: Can America and China Escape Thucydides’s Trap?

Amos Yadlin is former Chief of Israel’s Defense Intelligence and a Senior Fellow at Harvard University’s Belfer Center for Science and International Affairs.

Image: Reuters.