China’s military is gaining ground on the United States and threatening to overpower Taiwan, according to an unprecedented warning from Japanese defense officials.

“Therefore, it is necessary that we pay close attention to the situation with a sense of crisis more than ever before,” Japan’s annual defense white paper states. “The overall military balance between China and Taiwan is tilting to China’s favor, and the gap appears to be growing year by year.”

Japan has never mentioned the island democracy in a defense white paper, with past silence partly due to China’s irritation over international statements that fail to endorse Beijing’s claim to sovereignty over the island democracy. Yet, military strategists from Beijing and Tokyo, as well as Washington, view Taiwan as a choke point in the struggle for power in the Pacific, particularly with major competition between the U.S. and China underway.

“As China rapidly enhances its military power, changes in the military power balance between the United States and China may possibly affect the peace and stability of the Indo-Pacific region,” reads the Japanese paper. “It is necessary to pay greater attention to the military trends of the two countries in areas such as the South China Sea and Taiwan.”

As Japanese leaders grow more candid about threats from China, officials in Beijing have responded by citing Tokyo’s aggression during World War II to cast Japan as a villain in the modern-day dispute. Beijing has not governed the island of Taiwan since a fading Chinese imperial dynasty lost a war with Japan in 1895. After the Second World War, the island became the last refuge of the government overthrown during the Chinese Communist revolution.

“Recently, the Japanese side has been making issues out of China, grossly interfering in China’s internal affairs, making groundless accusations of China’s normal national defense and military activities, pointing fingers at China’s legitimate maritime activities and playing up the so-called ‘China threat,’” said Chinese Foreign Ministry spokesman Zhao Lijian on Tuesday. “China must and will be reunified. A complete reunification of China is most beneficial to regional peace and stability.”

Taiwan’s place on the map, south of Japan but north of other allies in the Indo-Pacific, means its fall to mainland forces would allow Beijing to drive a wedge between Japan and the American military bases in Okinawa from other U.S. and allied forces in the region.

“China has further intensified military activities around Taiwan including Chinese aircrafts entering the southwestern airspace of Taiwan. In the meantime, the United States has demonstrated a clear stance of supporting Taiwan in military aspects, such as transits by U.S. vessels through the Taiwan Strait and weapon sales,” the Japanese defense ministry white paper states. “Stabilizing the situation surrounding Taiwan is important for Japan’s security and the stability of the international community.”