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.”

This essay was originally published in Foreign Affairs magazine.

This is a welcome development for Beijing. Chinese analysts view South Korea as a weak link in the United States’ East Asian alliances, and Beijing is trying to divide Washington and Seoul through a combination of compliments and threats. In August 2020, Chinese media praised South Korea’s efforts to "be objective and keep its friendship with China," and several weeks later, Chinese scholars commended South Korea’s “kindness to China” in a time of “U.S. suppression.” But after South Korean President Moon Jae-in discussed Taiwan with Biden at their May 2021 summit, China’s Foreign Ministry warned South Korea not to “play with fire.” It is telling that Chinese scholars at a government-affiliated institute are arguing openly that China needs to raise the cost of South Korea’s cooperation with the United States on Taiwan.

North Korea’s missile capabilities are helping accomplish this task. The newer rockets more effectively threaten South Korea, and they increase Seoul’s doubts about the efficacy of U.S. deterrence. North Korea has explicitly tied its menacing assets to the issues surrounding the island. Pyongyang has publicly criticized the United States policies’ on Taiwan and threatened that “tragic consequences” will result from U.S. support. “The indiscreet meddling by the U.S. into the issue of Taiwan entails a potential danger of touching off a delicate situation on the Korean peninsula,” North Korea’s vice foreign minister said in a statement. These words could make Seoul think twice about backing the United States in the Taiwan Strait.

Japan is more difficult to split from Washington. But North Korea’s activities can certainly draw some of Japan’s attention away from Beijing. Although Tokyo was getting onboard with playing a greater role in deterring China and defending Taiwan, Japan’s Ministry of Defense has identified North Korea’s military capabilities as a “grave and imminent” threat, and there is no doubt that the government’s focus will shift if Pyongyang escalates its provocations. In the White House’s statement regarding Biden’s January 21 meeting with Japanese Prime Minister Fumio Kishida, Taiwan was mentioned only once. In comparison, the two leaders cited North Korea three times, condemned the country’s recent missile tests, and committed to work with South Korea more closely. Although Tokyo could theoretically focus on both North Korea and China, in practice it might struggle. Pyongyang poses a far more direct threat to Japanese lives and territory than does Beijing, and it would be hard for Japanese leaders to concentrate on China if North Korea grows more belligerent.

Drawing the Heat

North Korea’s new capabilities don’t help Beijing just diplomatically. The tests provide tangible, military benefits. The United States has been attempting to enlist South Korea in its efforts to strengthen deterrence across the Taiwan Strait. But Pyongyang’s new missiles mean Seoul is less likely to focus its military somewhere other than North Korea, especially if it continues the provocations. Indeed, a South Korean expert on Chinese politics has argued that when Washington asks for support in its contest with Beijing, Seoul should explain that it is too busy handling Pyongyang.

To reassure its allies, the United States may also need to refocus military attention on the Korean Peninsula, reducing its ability to operate in other parts of Asia. In 2017, when North Korea conducted ICBM and nuclear tests, the United States responded by sending more strategic assets, including heavy naval power, near the Korean Peninsula. If tensions rise high enough, Washington may have to do so again, including by shifting the Seventh Fleet’s operational focus to the area. Stationed in the middle of Japan, this fleet has been one of the United States’ primary tools for deterring Beijing, conducting patrols near the Taiwan Strait and promoting freedom of navigation in the South China Sea. But given past positioning patterns, a crisis on the Korean Peninsula would also most certainly drag the fleet farther north, undermining Washington’s ability to carry out operations elsewhere.

A major war on the Korean Peninsula would prove particularly devastating to the United States’ competition with Beijing. In addition to the Seventh Fleet, the United States Forces Korea’s 28,000 soldiers, 40 F-16 fighters, 90 military aircrafts, 40 attack helicopters, and other assets would immediately become unavailable for operations beyond the peninsula. A majority of the United States Forces Japan’s aircraft, ships, and approximately 55,000 military personnel would also be deployed to Korea. Japan’s own military, which could help the United States if it needed to fight China, would grow busy providing combat support to protect U.S. naval forces—including antisubmarine operations and sea minesweeping—as U.S. troops prepared for an amphibious landing on the peninsula.

Beijing, by comparison, is in a better position. The United States has to worry that China will use a North Korea–spurred crisis to invade Taiwan, but the inverse isn’t true: Beijing isn’t concerned that Seoul or Washington will start a war over Taiwan if Pyongyang launches an attack. China’s commitment to North Korea is also not as comprehensive as the United States’ is to Seoul. In the event of a renewed Korean war, China plans to send mostly ground forces into the North. Its air and naval assets would remain focused across the Taiwan Strait.

For China, therefore, a crisis on the peninsula—especially one that evolves into a conflict—would be a golden opportunity to expand its power. It may even make it possible to defeat Taipei. With U.S. intelligence assets supporting troops in Korea, a Chinese amphibious force might be able to move on the island without giving the United States advanced warning. China could establish beachheads on Taiwan long before U.S. forces, bogged down on the peninsula, have time to arrive. The war’s eventual outcome would be a fait accompli.

North Korea’s latest tests may have already made a Chinese attack more likely. As Chinese media happily pointed out, Pyongyang’s January 11 missile launch briefly confused the United States Northern Command’s warning system, grounding some commercial airplanes for 15 minutes. China has the most advanced ballistic and cruise missile program in the world. If North Korea’s offensive strike capability can jeopardize the U.S. early warning system, it surely bodes well for Beijing’s ability to surprise and defeat Washington’s forces.

Better Together

To counter North Korea’s new missile threats and prevent them from helping China, the Biden administration needs a stronger North Korea strategy—one that deters further provocations, reassures South Korea, and demonstrates Washington’s continued resolve and credibility to Beijing. That means Washington must support South Korea’s efforts to advance its offensive capabilities, such as the development of nuclear-powered submarines. South Korea, meanwhile, must scale up its combined exercises with the United States. A stronger U.S.–South Korean alliance will improve the two countries’ combat readiness, which is especially critical at a time when North Korea appears to be building up to another round of ICBM and nuclear tests. Finally, closer ties would make it easier for the United States to marshal allies in its competition against China, including in the Taiwan Strait.

The United States should also use the renewed tensions on the Korean Peninsula to encourage closer Japanese–South Korean cooperation. Seoul has long had highly fraught relations with its former colonial ruler, and the two states have especially struggled to get along in recent years. But for better or worse, the Korean Peninsula, East China Sea, and Taiwan Strait are increasingly intertwined in the current era of strategic competition. Pyongyang's provocations against the United States and its allies on the peninsula can embolden the Chinese Communist Party to act in other regions. And if Beijing can weaken or defeat the United States and its Asian allies anywhere, both the Chinese Communist Party and the Kim regime will be emboldened to act on the peninsula. To cope with this changing security environment, it makes sense for strategists in Washington, Seoul, and Tokyo to package these issues together. By demonstrating greater coordination, the three countries would also make it harder for China or North Korea to fracture Washington’s East Asia alliances, regardless of the contingency.

Finally, these three states must prepare for simultaneous provocations in East Asia, including concurrent conflicts in Taiwan and on the Korean Peninsula. In consultation with one another, the United States and its allies must demonstrate a strong willingness to cooperate and take strategic risks. They should hold more trilateral defense minister meetings, more thoroughly review various contingency scenarios, and discuss how to enhance their combined capabilities. Hopefully, these countries will never need to put these plans and abilities into practice. But to deter Kim Jong Un and Xi Jinping, they need to prove that they can fight two wars—and win both—if the need arises.