Throughout the Cold War, the great powers employed direct forces in only a handful of instances. Korea was the only conventional engagement from either bloc. The Soviets conducted limited actions within the Eastern bloc, most notably in Hungary and Czechoslovakia. But the Soviet invasion of Afghanistan and the U.S. intervention in Vietnam were both distinctly unconventional wars. American operations in Grenada and Panama, like Soviet deployments throughout Africa and the Middle East, were limited in scope and intensity.

By contrast, the Russian invasion of Ukraine is a massive conventional offensive, with its military component supporting maximal political goals—the Ukrainian regime’s destruction and replacement with a puppet government and likely the annexation of southern Ukraine. Russia seems unlikely to achieve its maximalist political goals without an open-ended force commitment, one that could push Russian society to the breaking point. The Kremlin may shift tack, seeking a settlement that gives it preference in or control over southern Ukraine and some guarantee against Ukrainian membership in the North Atlantic Treaty Organization.

Regardless of the outcome, Moscow’s actions demonstrate that force is the final arbiter between nations. Force is particularly viable in situations where states are beyond a clear security agreement. Taiwan is the clearest state in this circumstance. Like Ukraine, it is affiliated with American security structures but formally outside them. Unlike Ukraine, Taiwan has no strategic depth. Also unlike Ukraine, Taiwan is crucial to the global economic order—its semiconductor production sustains high-technology production internationally. That last consideration speaks directly to Taiwan’s defense: A ruinous war around the island would trigger economic effects that make the Ukraine crisis look like a daily stock-market dip. Nevertheless, with great-power force having been resurrected as a governing norm in international affairs, we should expect its increased use.

This is particularly crucial when considering Beijing’s interests and actions. China, or more specifically Xi Jinping’s Chinese Communist Party, has made its long-term intentions toward Taiwan clear. Mr. Xi sees a reclaimed Taiwan as the crown jewel in his legacy, the best way to solidify his role as the man responsible for reviving China as a great power and setting it on the path to global dominance. Even if NATO proves diffident in Ukraine, we can expect a strong Western defense of Taiwan because of the latter’s economic importance and centrality in America’s Indo-Pacific defense posture. Even so, we can expect Mr. Xi’s China to modify its calculus, recognizing that the Russian invasion of Ukraine might have a desensitizing effect and make an attack on Taiwan less shocking than it would otherwise be.

The broader question is of the organization of Eurasian security. One would expect the Russian invasion to formalize the return to traditional great-power politics, what theorists of international relations call “multipolarity,” a system in which multiple political and military centers of gravity exist. Russia, China and the U.S.—and perhaps Europe, depending on the Ukraine question’s settlement—can be expected, so the argument goes, to secure their own “spheres of influence,” dominating specific regions of the world and ceding others. The world, we will hear, is returning to the 19th century, albeit absent that era’s overwhelmingly Eurocentric geostrategic rhythm.

This prediction is alluring and wrong. Russia’s invasion of Ukraine has little impact on Chinese incentives, and China remains the crucial actor. The Communist Party under Mr. Xi—and perhaps since Deng Xiaoping, depending on how politically sophisticated it is—drew a unique lesson from the Soviet collapse. The Soviets failed not because they didn’t integrate capitalist insights into their economy, but because they never went far enough in their external expansion.

Soviet Russia failed to grasp that it couldn’t coexist with any other power. The only way for an imperial dictatorship like the U.S.S.R. or Nazi Germany to survive is through absolute domination. Soviet leaders after Stalin progressively lost sight of this reality, and by Leonid Brezhnev’s death in 1982 they had become reactive to an increasingly assertive Western military-strategic approach.

The world has many dictatorships—the Kim family’s in North Korea, the increasingly kleptocratic Iranian theocracy and the Castro regime in Cuba, to cite obvious examples. But none of these are nearly big and powerful enough to be structurally disruptive in the way the Soviet Union and Nazi Germany were—or China is today. Some sort of external economic dependence is inevitable, even if only on an autarkic bloc. And that bloc, Chinese leaders realized, is naturally brittle. Internal dissent can be managed only if the world at large is properly ordered. Hence the Communist Party’s drive for absolute dominance.

Assuming Russia’s collapse is not imminent, China will use Russia’s increasing isolation to transform Moscow into a petrochemical satellite, taking advantage of Western sanctions to secure Russian energy flows indefinitely. In turn, China hopes that Russia, humbled or emboldened by its Ukraine adventure—and with or without Mr. Putin at the helm—will occupy Western attention as Beijing gobbles up the choicest Pacific possessions and extends its economic and diplomatic tendrils into the Middle East, Africa and Eastern Europe. Far from accepting independent Russian action, China is counting on Russian failure to accelerate the satisfaction of its boundless appetite.

That will create two blocs, not three. On the one side will stand the U.S. and its allies, on the other China and its affiliates and satellites. War between the two is all but inevitable. The U.S. must take note: Triangulation against China is impossible with Russia in an abject state of economic dependence. The large strategic issue in the Ukraine war is the possibility of China’s domination of Eurasia.