Updated August 2, 2021 at 12:13 PM ET

SEOUL, South Korea — Japan is undergoing a remarkable shift in its stance on one of the most contentious issues in Asia: Taiwan.

Mainland China and Taiwan split during a civil war in 1949, and Beijing has vowed to unify with the self-governing island — by force, if necessary. The Biden administration is counting on help from its allies, especially Japan, to deter such a move.

For decades, Japan considered the Taiwan issue too politically sensitive to speak out about it publicly. Japan's military is focused on defense of its own territory and has no expeditionary forces to fight overseas.

But in recent weeks, top Japanese officials have said that if mainland China attacks the island, Japan should join the U.S. in defending it.

"We have to protect Taiwan, as a democratic country," Japan's deputy defense minister, Yasuhide Nakayama, said in a conference in June.

Japan's shift in thinking comes as China has stepped up pressure on Taiwan, including sending fighter jets and warships around the island. But the bolder talk could also be driven by further moves by China.

Yoji Koda, a former commander of Japan's Maritime Self-Defense Force fleet, says he believes Tokyo's trust in Beijing is eroding. He points to China's rapid military buildup, its crushing of dissent in Hong Kong and its flouting of an international court ruling that rejected China's claim over the South China Sea.

Tensions have also mounted between Japan and China over disputed territory known as the Senkaku Islands in Japan and the Diaoyu Islands in China.

As neighbors, Koda says, "we need to say what we think."

Making the case for a security threat

The Japanese Constitution rejects using force to resolve international disputes. But after 2015 reforms, Japanese law allows the military to use force when an attack on a foreign country threatens Japan's survival. The law also would let Japan deploy its forces to provide logistical support to foreign militaries ensuring Japan's security.

In early July, Deputy Prime Minister Taro Aso reiterated that any crisis over Taiwan should be resolved through dialogue.

But speaking at a fundraising event, he said, "If a major problem took place in Taiwan, it would not be too much to say that it could relate to a survival-threatening situation" for Japan.

The Defense Ministry issued a white paper in July that said, "Stabilizing the situation surrounding Taiwan is important for Japan's security." It said Japan should monitor the situation "with a sense of crisis."

These statements signal that Japan is building an argument that an attack on Taiwan could meet Japan's conditions for activating its military, analysts say.

"It's that public connection now with Taiwan — that is the part that's new," says Jeffrey Hornung, a political scientist at Rand Corp.

Japan's official policy still recognizes the authorities in Beijing, not Taipei, as China's legitimate government. And China and Japan are major trading partners.

That has not changed. But Japan's new messaging has irked Beijing, which has criticized it as dangerous. The Chinese Foreign Ministry has repeatedly said that China won't let anyone stand in the way of its efforts to unify with Taiwan.

China looming in meetings

The bolder tone in Tokyo has also followed high-level meetings in which China loomed large.

In mid-March, the U.S. secretary of state and secretary of defense went to Japan.

Then at a summit in April in Washington, D.C., Japanese Prime Minister Yoshihide Suga and President Biden became their countries' first leaders in 50 years to mention the Taiwan issue in a joint statement. The next month, Suga issued another joint statement with European leaders. Both statements stressed the need for "peace and stability across the Taiwan Strait."

China, for its part, has said U.S. warships in the region are "the biggest destroyer of peace and stability."

Close neighbors

Looking at a map, Taiwan's importance to Japan is hard to miss.

Waterways between them are strategic choke points that could be used to control travel and shipping through the region.

Japan's Yonaguni island lies less than 70 miles off Taiwan's east coast. Yonaguni is part of an archipelago that's administered by Okinawa prefecture — where 70% of U.S. military bases in Japan are located.

Koda and other analysts believe China might attack Yonaguni and possibly other nearby Japanese islands to control approaches to Taiwan.

Rand Corp.'s Hornung says Japan could do many things, short of sending troops, to help the U.S. in case of an invasion of Taiwan, such as intelligence, reconnaissance, "defense of U.S. bases, defense of Japanese waters, like defending choke points or defending airspace."

Japan should insist on avoiding war

Kyoji Yanagisawa, a former deputy defense minister, acknowledges that Japan can't avoid involvement. "As long as the U.S. military uses Japanese bases to launch attacks, Japan will certainly be affected in the event of an emergency in Taiwan," he says. "Sooner or later, a Taiwan emergency will turn into a Japan emergency."

The problem, he says, is that Japanese officials are now thinking more about how to win a conflict than how to avoid one in the first place.

He believes Japan's government now appears to have completely sided with the U.S. in its dispute with China. But he cautions that the more that ties between Beijing and Washington deteriorate, the more Tokyo needs to keep open lines of communication to both governments.

"Given its position," he argues, "Japan should insist that the U.S. avoid anything that could lead to war. At the same time, Japan should insist on the same from China."

But Koda, a retired vice admiral, argues that Japan must also prepare for a worst-case scenario. He expects the U.S. and Japan to draft an operational plan for a Taiwan conflict within the next year or so.

If Japan fails to do so, Koda says, the "Japanese government would be called the most stupid government in Japanese history."

Perhaps not now, he adds, but by historians and strategists in centuries to come.

Chie Kobayashi contributed to this report from Tokyo.

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