Between China’s 20th Communist Party Congress, that began on Sunday, and the next one in 2027, Japan will undertake its biggest arms buildup since World War Two in a race to deter Beijing from war in East Asia, according to Japanese government officials and security analysts.

Japan identified China as its chief adversary in its 2019 defence white paper, worried that Beijing’s flouting of international norms, pressure on Taiwan and rapid military modernisation posed a serious security threat. That anxiety has intensified since Russia invaded Ukraine, weakening Japanese public opposition to rearming, security experts say.

By pointing to 2027 as the moment when East Asia’s power balance may tip in China’s favour, Japan’s government can rally support for greater defence spending, according to a professor. In addition to being the next time Communist Party delegates gather in Beijing, 2027 is the next major milestone on China’s military modernisation roadmap and the centennial of the founding of the People’s Liberation Army. At a congressional hearing last year, US Indo-Pacific commander Admiral Philip Davidson said that China’s threat against Taiwan could “manifest” that year.

For Japan, losing Taiwan to mainland Chinese control could be a disaster because it would jeopardise key shipping lanes that supply nearly all Japan’s oil and many of the materials it uses for manufacturing. It would also give the Chinese navy unfettered access to the Western Pacific from bases on the island.

China’s foreign ministry said Japan was using China as a pretext for a military buildup.

Japan’s delicate diplomatic and economic relations with its bigger neighbour mean it is unlikely to commit to directly defending Taiwan. But with Japan’s nearest territory only about 150 kilometres (93 miles) from the island, it could be drawn into conflict with an adversary that spends more than four times as much on its military.

China could try to capture Japanese islands close to Taiwan to establish air defences and fend off any counter attack, said another Japanese government official involved in planning, who also asked not to be named because he is not authorised to talk to the media..

Japanese military bases, airports, seaports, and other logistical hubs could also be tempting targets for Chinese missile strikes because they would be staging grounds for US forces.

Russia’s invasion of Ukraine, which it calls a “special operation”, has helped shift public opinion in Japan away from the postwar pacifism that has dominated defence policy for decades.

In an opinion poll published by public broadcaster NHK this month, 55% of 1,247 people surveyed said they supported increased defence spending, compared with 29% who opposed it.

In July, Prime Minister Fumio Kishida won national upper house elections with a pledge to “substantially” increase defence spending. His ruling Liberal Democratic Party promised to double the military budget to about 10 trillion yen ($68 billion) within five years.

Japan’s focus on China is unlikely to waver, analysts say, even as its former top adversary, North Korea, is in the midst of a fresh cycle of missile tests, the latest on Friday, including the first flight over Japan since 2017. After the Chinese Communist Party congress, the Kim Jong Un regime is widely expected to follow up with a nuclear test.

Japan wants to let South Korea take the lead in tackling its belligerent northern neighbour, a senior Japanese Self Defence Force commander said, speaking anonymously because of the sensitivity of the matter.

“I don’t see North Korea’s actions leading to any significant change” to Japan’s China focus,” said Bonji Ohara, senior fellow at the Sasakawa Peace Foundation and a former military attache at Japan’s embassy in China. North Korea’s latest actions may even help solidify general support for it, he added.