A large portion of global trade passes through the South and East China Seas. Chokepoints like the Bashi Channel sweep Taiwan’s edges.

A successful Chinese conquest of Taiwan would enable Beijing to dominate the region’s strategic waterways, project military power widely into the Pacific and more aggressively pursue its contested maritime and territorial claims.

“The balance of power in Asia would be tipped quite decisively in favor of China should Taiwan fall into China’s hands,” said Robert Ward, Japan chair at the International Institute for Strategic Studies.

Looking seaward, China is “sort of hemmed in” right now, he said, referring to the First Island Chain—a string of archipelagoes off the country’s east coast made up largely of a trio of U.S. partners: Japan, Taiwan and the Philippines. China “clearly wants to break out of that,” Ward said. 

In a conflict, Taiwan’s fate would become quickly intertwined with the U.S.-Japan security alliance. To repel a full-scale attack, Taipei would need America—its main defense partner—to join the fight. To fight effectively, American forces would need Japan. 

Whether the U.S. would intervene directly in a clash is an open question. Washington maintains a policy of what it calls “strategic ambiguity” to keep China guessing.

Geography presents particular challenges for Japan. A sweep of Japanese islands called the Ryukyus arcs southwest, stopping just short of Taiwan. The island of Yonaguni, more than 1,200 miles from Tokyo, is less than 70 miles from Taiwan.

Some of these islands would be right next to the war zone—or even in it—if China sent missiles and warships to blockade, batter and encircle Taiwan, putting Japanese citizens and territory at risk. Hostilities across otherwise busy shipping lanes would disrupt essential trade Japan relies on.

In recent years, as Tokyo has retooled its security posture in response to China’s rapid military rise, it has sent a flurry of investments to these southwestern islands. That includes new bases, radar facilities, electronic-warfare capabilities and missile systems. 

Japan’s Type 12 antiship missile batteries with a 125-mile range are now positioned on a number of islands in the Ryukyus, and longer-range variants are being developed. Japanese officials say Yonaguni will receive surface-to-air missiles. 

If Japan decided to participate in a conflict over Taiwan, these missiles—and others it is making to hit targets further out—would play an important role, as would Japan’s submarines and warships.

Japan has its own territorial dispute with China over a cluster of uninhabited islands called the Senkakus. Controlled by Japan and claimed by China and Taiwan, these specks of land lie just over 100 miles northeast of Taiwan. China, which calls them the Diaoyu islands, routinely sends coast-guard ships into the waters around them to assert its claim, a source of friction with Tokyo.

A Chinese takeover of Taiwan would make the threat to Japanese territory more acute, said Yuki Tatsumi, senior director at the U.S.-based Institute for Indo-Pacific Security. “Japan will no longer have a buffer, if you will,” she said, adding that it would face direct pressure from the Chinese navy.

Whether or not Japan joins a war over Taiwan, to what degree it fights and what it does if the U.S. stays out depend on a mix of military, legal and political factors. If Washington decided to militarily defend Taiwan, decisions made in Tokyo would shape the U.S. intervention. 

Japan hosts a network of strategically positioned American bases, sustained by robust infrastructure. U.S. forces fighting a long way from home would need to activate them, involving discussions between Washington and Tokyo. That includes Kadena Air Base, a hub of U.S. air power in the Pacific, and the U.S. Navy base at Yokosuka, home to the Seventh Fleet and a forward-deployed aircraft carrier.

“The U.S. doesn’t really have anything like the bases that it has in Japan elsewhere in Asia,” said Eric Heginbotham, a principal research scientist at the Massachusetts Institute of Technology’s security studies program. “You can’t replicate what you have in Japan.”

A 2023 report on a Taiwan wargame by the Center for Strategic and International Studies, co-written by Heginbotham, described Japan as the linchpin. “The United States must be able to use its bases in Japan for combat operations,” it said. 

One island in particular, Okinawa, is blanketed with military facilities and training areas. Kadena Air Base, located on it, houses a range of aircraft at any given time, including jet fighters that would be needed to sink Chinese ships in the Taiwan Strait and fight Chinese warplanes in the air. A new missile-toting Marine Corps unit designed for an island-hopping fight is also based on Okinawa.

Okinawa’s proximity to China, however, makes it vulnerable to attack. Beijing has amassed such a formidable arsenal of missiles that concentrating combat planes in one or two well-known locations almost anywhere in the region would put them in danger of being wiped out. To make it harder for China to target them, the U.S. Air Force would, in a conflict, seek to scatter its aircraft not just across various American bases but beyond: to Japanese military installations, civilian or dual-use airfields in Japan, and sites elsewhere in the region. 

That means U.S. air operations could expand Japan’s involvement, if Tokyo acquiesced.

Japan would be especially critical for fighter aircraft, which have limited range and burn a lot of fuel. From runways in Japan, pilots could get to Taiwan and back quickly, giving them more time in the fight rather than the commute. 

Without Japan, the U.S. would be heavily reliant on Guam, an American territory in the West Pacific around 1,700 miles from Taiwan. Bombers, which fly long distances, could use Guam, but fighter operations would be much harder to sustain, even with aerial refueling. Air-superiority missions, for instance, would require them to sortie and remain “on station” in Taiwan’s vicinity for a time before returning, which isn’t viable from Guam, Heginbotham said. 

Aircraft carriers would be an option, but those warships are big, expensive targets and the U.S. is unlikely to put too many of them forward in the early days of a conflict, he said. 

Naval bases on Japan, meanwhile, would help rearm warships. The farther vessels have to go to pick up new missiles, the longer they would be out of the fight. 

Japan could also be dragged in by China’s wartime decisions. If Beijing calculated that the U.S. was likely to join the conflict, it could pre-emptively seek to blunt that threat by sending missile barrages at American bases in Japan, and potentially at Japanese targets as well. It has hundreds of missiles in each category—short, medium and intermediate range.

Missiles are proliferating on all sides. This year, the U.S. for the first time brought its Typhon system to Japan—it has since left the country—placing it in the south from where it could hit targets on mainland China. The new ship-killing Nmesis missile battery used by the U.S. Marine Corps was also taken to Okinawa for exercises.

Write to Niharika Mandhana at niharika.mandhana@wsj.com and Daniel Kiss at daniel.kiss@wsj.com