Russia is upping its World War III rhetoric. Vladimir Putin has threatened any nation that directly intervenes in Ukraine with retaliation via "strategic" weapons — "strategic" meaning "nuclear." At the same time, Russian foreign minister Sergei Lavrov says the risks of a nuclear war are now "very significant."

On Russian state media, prominent commentators this week suggested that a nuclear war with the West wouldn't be too problematic because Russians would go to heaven, whereas Westerners would simply perish. The fact that this was allowed on state media is also highly significant, and obviously, there is nothing more serious than nuclear war.

Yet this is not the time to bow before Russian threats. Indeed, Biden must respond to Russian aggression forcefully. As Americans understandably look with concern at Russia's escalation, they must also be reminded that this showdown is not simply about whether Ukraine survives as a sovereign democracy or becomes a Russian puppet state. This conflict is about the future of the entire free world.

Ukraine's survival matters for reasons that go beyond its immediate moral and political merits. This moment is also shaping a message about what America, and by association what the free world, will tolerate in the 21st century. A sacred principle of the post-1945 democratic international order is that a sovereign democracy cannot be extinguished simply because a more powerful nation desires it. If Biden wavers, he will be sacrificing the relative peace and cooperation that was hard-won in World War II.

Putin is gambling that the West is too individualistic, selfish, and weak to match its previous successes. He gambles that Americans now lack the spirit that allowed us to win both a hot war and a Cold War. He gambles that even as the West's economic and political power has soared, its moral resolve has rotted.

Now finding his thesis challenged by Western resolve over Ukraine, Putin has resorted to the ultimate escalation: the dangled dagger of nuclear holocaust. This is the apex threat of what Carl von Clausewitz called "absolute war," the realm of war in which politicians, civilians, and military blur into one — where Joe, Jamal, and Jenny have as much to lose from war as Joe Biden, Kamala Harris, and Mark Milley. The question follows: Do Joe, Jamal, and Jenny, like their forebears in World War II, share a willingness to stare down the threat and win?

They face a defining choice: whether to sacrifice the free world that America built after World War II or whether to defend it. This is not a question of offensive democracy, as in Iraq. It is a question of defense. The courage of Ukraine's own people made their nation a democracy. If we yield Ukraine, what follows next?

If Putin succeeds in this aggressive action, he will surely expand his new Russian imperium beyond Russia, Belarus, and Ukraine. He is already moving on Moldova. And if he cows the United States into retreat, he will next replicate his nuclear blackmail so as to delete the Baltic democracies. Putin would own the red line marker.

And Xi Jinping's China would sense that its own opportunity had arrived. Only communist China has the economic power and political ambition to replace the U.S.-led democratic international order with its own feudal, authoritarian order. And why, after Putin's successful nuclear gambit over Ukraine, wouldn't Xi move to trample Taiwan? If he were to fail against a poorly prepared U.S. Navy, Xi could simply follow Putin's example and hold the West hostage with nuclear blackmail. Even the most cursory look at China's state media and rhetoric shows its desperate hope for Russia to avoid defeat. Xi sees Putin's invasion of Ukraine as a test for his own agenda.

At a minimum, a U.S. retreat from Ukraine would give rise to a world in which nuclear blackmail works. Iran and North Korea would use their nukes to test a depleted American deterrence. Their adversaries (Saudi Arabia, the United Arab Emirates, Japan, and South Korea) would be forced to develop their own nuclear weapons outside of the traditional U.S. security orbit. The risk of miscalculation and uncontrolled proliferation would grow in tandem.

This is why the answer is not to capitulate, but to push back as hard as possible. The Biden administration must start by reversing its nuclear appeasement policy toward Moscow. Immediately recommence normally scheduled missile tests. Remind Putin in bright, bold letters that the choice to use nuclear weapons will end his regime. Biden must not forget the principle that has prevented nuclear war since 1945 — namely, the fact that even though anyone can launch a few nukes, no one can survive American nuclear retaliation.

By destiny and history, Biden leads the world at a critical moment. Demanding more of his allies, he must pass this 21st-century test for freedom, proving to Putin that escalation will lead only to Russia's doom.