The U.S. and China are engaged in a “strategic competition,” as the Biden administration has put it, with Taiwan emerging as the focal point. But an ascendant view inside the administration seems to be that while China represents a serious economic, political and technological challenge to American interests, it doesn’t pose a direct military threat. This is a very imprudent assumption that could lead to war and, ultimately, American defeat. To avoid that disastrous outcome, the U.S. must recognize that China is a military threat—and conflict could come soon.

What...

Illustration: David Klein

The U.S. and China are engaged in a “strategic competition,” as the Biden administration has put it, with Taiwan emerging as the focal point. But an ascendant view inside the administration seems to be that while China represents a serious economic, political and technological challenge to American interests, it doesn’t pose a direct military threat. This is a very imprudent assumption that could lead to war and, ultimately, American defeat. To avoid that disastrous outcome, the U.S. must recognize that China is a military threat—and conflict could come soon.

What makes China an urgent military threat? First, Beijing has made clear it is willing to use force to take Taiwan. Subordinating the island isn’t only about incorporating a putative lost province—it would be a vital step toward establishing Chinese hegemony in Asia. And this isn’t mere talk. The Chinese military has rehearsed amphibious attacks, and commercial satellite imagery shows that China practices large-scale attacks on U.S. forces in the region.

Second, China doesn’t merely have the will to invade Taiwan, it increasingly may have the ability to pull it off. China has spent 25 years building a modern military in large part to bring Taiwan to heel. China now has the largest navy in the world and an enormous and advanced air force, missile arsenal and network of satellites. This isn’t to say China could manage a successful invasion of Taiwan tomorrow—but Beijing could be very close. It will be “fully able” to invade by 2025, Taiwan’s defense minister said recently. China’s military power is improving every month.

Third, China may think its window of opportunity is closing. Many wars have started because one side thought it had a time-limited opening to exploit. Certainly this was a principal factor in the outbreaks of the two world wars. Beijing may reasonably judge this to be the case today.

The U.S. is finally, if too slowly and fitfully, waking up to the China challenge and reorienting its military efforts toward Asia. But these investments won’t really start to pay off until later this decade. Meanwhile, coalitions like the Quad (the U.S., Australia, Japan and India) are coalescing to deny China the ability to dominate the region. From Beijing’s view, if it waits too long, America’s military investments will yield a much more formidable opponent, while an international coalition works to frustrate Chinese ambitions.

This all adds up to a situation in which Beijing may reckon it would be better to use force sooner rather than later. To avoid a conflict, and possible defeat, the U.S. must act quickly to deter Beijing. Repeatedly declaring our “rock solid” commitment to Taiwan is fine but insufficient.

The most urgent priority: Taiwan must radically upgrade its defenses. The island’s own efforts in this regard will decide whether it survives as a free society. Taipei must multiply its defense budget, grossly neglected in recent decades, and focus its expenditures and efforts on two things: degrading a Chinese invasion with the help of the U.S. and making the island resilient to a blockade and bombardment by Beijing. This will require antiship missiles, sea mines and air defenses, as well as stockpiles of supplies to ride out a blockade. The U.S. will need to use every lever to prod or force Taipei to make this shift.

Washington should also bring comparable pressure on Japan, America’s single most important ally. If Taiwan falls, Japan will be under direct military threat from Beijing. And Japan would play a critical role in any defense of Taiwan. Japan should at least double its defense budget (now merely 1% of gross domestic product) immediately.

Meanwhile, the U.S. needs to strengthen its military position west of the international date line. A potent forward-deployed force of Marines, submarines and other survivable forces would ensure America and its allies could blunt any attack against Taiwan. The U.S. must buy and rapidly field systems like antiship missiles and unmanned reconnaissance platforms that would be essential to defeating a Chinese invasion.

Averting war against a superpower will require being ruthless about American priorities, though. Holding the line in Asia will mean the U.S. military will have to stop doing almost everything else other than nuclear deterrence and counterterrorism. The U.S. military will have to scale down in the Middle East, Africa, Latin America and even Europe. America had a chance to make a more evolutionary and balanced shift to Asia, but we blew it. Now we need to focus, even if it means the military must effectively drop everything else.

China will surely pose a long-term challenge to the U.S. in areas outside the realm of military power. But the most pressing risk is that Beijing may see an advantage in resorting to war. Convincing Beijing it won’t gain from aggression must be the overriding priority.

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.” He served as deputy assistant secretary of defense for strategy and force development, 2017-18.

China’s hypersonic missile test demonstrates the next major war will utilize cyber attacks and unmanned vehicles striking from afar. So far the Biden administration is ignoring the warning signs. Images: EPA/Shutterstock/Getty Images Composite: Mark Kelly The Wall Street Journal Interactive Edition