This commentary was originally published by The Wall Street Journal.

A Russian invasion of Ukraine would be the most consequential use of military force in Europe since World War II and could put Moscow in a position to threaten U.S. allies in Europe. Many in the American foreign-policy establishment argue that the appropriate U.S. response to any such invasion is a major American troop deployment to the Continent. This would be a grave mistake.

The U.S. can no longer afford to spread its military across the world. The reason is simple: an increasingly aggressive China, the most powerful state to rise in the international system since the U.S. itself. By some measures, China’s economy is now the world’s largest. And it has built a military to match its economic heft. Twenty-five years ago, the Chinese military was backward and obsolete. But extraordinary increases in Beijing’s defense budget over more than two decades, and top political leaders’ razor-sharp focus, have transformed the People’s Liberation Army into one of the strongest militaries the world has ever seen.

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China’s new military is capable not only of territorial defense but of projecting power. Besides boasting the largest navy in the world by ship count, China enjoys some capabilities, like certain types of hypersonic weapons, that even the U.S. hasn’t developed.

Most urgently, China poses an increasingly imminent threat to Taiwan. Xi Jinping has made clear that his platform of “national rejuvenation” can’t be successful until Taiwan unifies with the mainland—whether it wants to or not. The PLA is growing more confident in its ability to conquer Taiwan even if the U.S. intervenes. Given China’s military and economic strength, China’s leaders reasonably doubt that the U.S. or anyone else would mount a meaningful response to an invasion of Taiwan. To give a sense of his resolve, Mr. Xi warned that any “foreign forces” standing in China’s way would have “their heads . . . bashed bloody against a Great Wall of steel forged by over 1.4 billion Chinese people.”

The U.S. must defend Taiwan to retain its credibility as the leader of a coalition for a free and open Indo-Pacific. From a military perspective, Taiwan is a vital link in the first island chain of the Western Pacific. If Taiwan falls into Chinese hands, the U.S. will find it harder to defend critical allies like Japan and the Philippines, while China will be able to project its naval, air and other forces close to the U.S. and its territories. Taiwan is also an economic dynamo, the ninth-largest U.S. trading partner of goods with a near-monopoly on the most advanced semiconductor technology—to which the U.S. would most certainly lose access after a war.

The Biden administration this month ordered more than 6,000 additional U.S. troops deployed to Eastern Europe, with many more potentially on the way. These deployments would involve major additional uncounted commitments of air, space, naval and logistics forces needed to enable and protect them. These are precisely the kinds of forces needed to defend Taiwan. The critical assets—munitions, top-end aviation, submarines, and intelligence, surveillance and reconnaissance capabilities—that are needed to fight Russia or China are in short supply. For example, stealthy heavy bombers are the crown jewel of U.S. military power, but there are only 20 in the entire Air Force.

The U.S. has no hope of competing with China and ensuring Taiwan’s defense if it is distracted elsewhere. It is a delusion that the U.S. can, as Pentagon press secretary John Kirby said recently, “walk and chew gum at the same time” with respect to Russia and China. Sending more resources to Europe is the definition of getting distracted. Rather than increasing forces in Europe, the U.S. should be moving toward reductions.

There is a viable alternative for Europe’s defense: The Europeans themselves can step up and do more for themselves, especially with regard to conventional arms. This is well within Europe’s capacity, as the combined economic power of the NATO states dwarfs that of Russia. NATO allies spend far more on their militaries than Russia. To aid its European allies, the U.S. can provide various forms of support, including lethal weapons, while continuing to remain committed to NATO’s defense, albeit in a more constrained fashion, by providing high-end and fungible military capabilities. The U.S. can also continue to extend its nuclear deterrent to NATO.

The U.S. should remain committed to NATO’s defense but husband its critical resources for the primary fight in Asia, and Taiwan in particular. Denying China the ability to dominate Asia is more important than anything that happens in Europe. To be blunt: Taiwan is more important than Ukraine. America’s European allies are in a better position to take on Russia than America’s Asian allies are to deal with China. The Chinese can’t be allowed to think that America’s distraction in Ukraine provides them with a window of opportunity to invade Taiwan. The U.S. needs to act accordingly, crisis or not.

Ms. Mastro is a center fellow at the Shorenstein Asia-Pacific Research Center, part of Stanford’s Freeman Spogli Institute for International Studies, and a nonresident senior fellow at the American Enterprise Institute. Mr. Colby is a principal at the Marathon Initiative and author of “The Strategy of Denial: American Defense in an Age of Great Power Conflict.”