The United States has made a number of strategic miscalculations since Russia invaded Ukraine in 2022, but the single greatest may be the message that the Biden administration just sent about nuclear weapons. The U.S. showed that it would protect a nuclear-armed friend, Israel, from an as-yet-nonnuclear enemy (Iran); at the same time, Washington has refused to consider using its forces to defend a nonnuclear friend (Ukraine) against a nuclear-armed Russia.

Other governments will deduce that states with nuclear weapons can barbarically attack America’s friends and bully U.S. leaders into abandoning them. The British government has underscored that sentiment by basically admitting that, precisely because of fears of escalation with Russia, Ukraine won’t get the same help that Israel did. Even if the U.S. and its allies were more coy about their calculations, their conduct will encourage a wave of nuclear proliferation in the coming years.

Indeed, escalation worries have made the U.S. timid about helping Ukraine in ways far short of the direct defense it provided Israel. Even when grudgingly going along with Kyiv’s requests for advanced weaponry, the Biden administration has imposed limits on how and where that equipment can be used.