When Ukraine regained its independence after the dissolution of the Soviet Union in 1991, it had the world’s third-largest stockpile of nuclear warheads. Had it remained a member of the nuclear club, it would not have lost Crimea to a Russian invasion in 2014 and would not be fighting now for its life against a massive and illegal Russian onslaught.

But Ukraine gave up its nuclear weapons. Yielding to pressure from the United States and Britain, it signed an agreement in 1994 to turn over its arsenal to Russia. In exchange, Russia pledged to respect Ukraine’s independence, sovereignty, and existing borders. Russia also obligated itself “to refrain from the threat or use of force” against Ukraine and promised that no Russian weapons “will ever be used against Ukraine.”

With nothing to enforce them, however, those promises were worthless.

Ukrainians have inspired and united the civilized world with their tenacious resistance to Vladimir Putin’s unprovoked attack, a war of bloody aggression resembling the Soviet and German invasions of Poland in 1939. As of this writing, on Tuesday, Ukraine remains free. But a 40-mile-long convoy of Russian armored vehicles is heading toward the capital city of Kyiv, burning homes and buildings as it advances, and civilian areas of Kharkiv, Ukraine’s second-largest city, are being shelled indiscriminately by Russian rockets. It is far from clear that Ukraine, fighting alone against a nuclear power, can win this war.

And if Ukraine falls, will Taiwan be next?

On the day Russia launched its invasion, nine Chinese fighter jets invaded Taiwan’s air defense identification zone, forcing Taiwan’s air force to scramble its own fighters in response. Two days later, China struck again, sending six fighter jets and two anti-submarine aircraft over the southwest corner of Taiwan’s air-defense zone. China now does this routinely, deliberately acting to intimidate its small neighbor and keep it in a constant state of tension. On some occasions, it goes much further, firing missiles into the seas off Taiwan’s coast or staging simulated invasions of the island.

Repeatedly, the communist regime in Beijing claims that Taiwan — a nation of 23 million people that has never been ruled by the People’s Republic of China — has no right to an independent existence. It openly threatens to go to war to enforce its outrageous demand.

“We do not renounce the use of force” to prevent Taiwan from being recognized as a sovereign nation, President Xi Jinping of China has said. Under a sinister headline — “Time to warn Taiwan secessionists and their fomenters: War is real” — the Chinese Communist Party newspaper Global Times proclaimed last October that China’s armed forces are making “preparations based on the possibility of combat … to use force against Taiwan.” It vowed that there would be “military punishment” if Taiwan does not “reverse the current situation” and submit to China’s control. “This warning is not just a verbal threat,” the editorial ended.

With Russia engaged in a murderous assault on Ukraine, protecting Taiwan from a similar fate must be a top US priority. Under the terms of the Taiwan Relations Act, the United States is legally bound to provide Taiwan with all military equipment needed for its self-defense. But under the bizarre doctrine of “strategic ambiguity,” Washington has never explicitly committed to fighting alongside Taiwan should it be attacked.

That ambiguity should have been abandoned long ago. A successful Chinese invasion of Taiwan would be an international catastrophe. Besides shattering global trade, it would endanger the whole Western Pacific and, as former Japanese prime minister Shinzo Abecq said this week, directly endanger Japan as well.

Like Ukraine, Taiwan was once a member (or on the verge of membership) in the nuclear nations club. Like Ukraine, Taiwan was pressured by the United States into abandoning its nuclear ambitions. It is too late now to empower Ukraine with the nuclear deterrent that would have kept Russia at bay. But it’s not too late to do so for Taiwan.

To safeguard the island from the invasion that Beijing keeps threatening to unleash, Taiwan needs a nuclear arsenal ASAP. Even a relative handful of missiles with nuclear warheads would suffice to change Beijing’s calculus on Taiwan and deter an attack from the mainland. In much the way that the Reagan administration deployed nuclear-armed Pershing II and cruise missiles in Europe during the 1980s to deter a Soviet attack, the Biden administration should make nuclear missiles available to Taiwan now, making sure that China knows their purpose is to defend Taiwanese independence and sovereignty.

The regime in Beijing is evil but not irrational. It will not go to war to crush Taiwan’s sovereign democracy if it will face nuclear-armed defenders. Proliferating nuclear weapons to another country is always a cause for concern, but a Chinese conquest of one of Asia’s key democracies poses a far greater threat. Taiwan’s fate is on the line, and there is no more time to waste.

(Jeff Jacoby is a columnist for The Boston Globe).