Japan's nationalization of the Senkaku Islands in the East China Sea has sparked security tensions in the region for the past 10 years, prompting Tokyo, known for its pacifist Constitution, to be acutely wary of military threats from Beijing.

Japan has administered the Senkakus, but China has claimed the uninhabited islets since the early 1970s, calling them Diaoyu, after studies by the United Nations indicated there may be potentially lucrative gas reserves around them.

On Sept. 11, 2012, the Japanese government of then Prime Minister Yoshihiko Noda put the islands under state control, five months after then Tokyo Gov. Shintaro Ishihara abruptly announced the metropolis would buy some of the Senkakus from a Japanese private owner.

File photo taken from a Kyodo News airplane shows the China Coast Guard's Haijing 2350 (L) and the Japan Coast Guard's patrol ship Hateruma in Japanese territorial waters near Uotsuri, one of the five main islands in the Japanese-controlled Senkaku group, in the East China Sea on Sept. 10, 2013. (Kyodo)

Subsequently, Communist-led China has stepped up provocations in the nearby waters, frequently sending its coast guard vessels near the islets, destabilizing the regional security environment. Beijing has insisted the islands are its "inherent territory."

In Japan, worries are mounting that the leadership of Chinese President Xi Jinping could attempt to invade the islets after conquering self-ruled democratic Taiwan, regarded by Beijing as its province to be reunified with the mainland, by force if necessary.

Whenever their leaders are replaced, Tokyo, which has heavily depended on Washington for military protection, has always tried to receive confirmation from the United States that the Senkakus are covered by Article 5 of the 1960 Japan-U.S. security treaty.

"We cannot let our guard down against China, given its apparent ambition to change the status quo by force," a Japanese government official said. "The nationalization gave us a cue to seriously reconsider our defense policy with a high sense of urgency."

The Senkakus are under the jurisdiction of Japan's southern island prefecture of Okinawa -- a geopolitically important region that still hosts the bulk of U.S. bases in the country over 50 years after it was reverted to Japan in 1972 following U.S. rule.

During a campaign for the Okinawa gubernatorial election on Sunday, three candidates focused on security matters, including the long-running issue of the relocation of a key U.S. Marine Corps base that would work as a deterrent against China.

Although Japan's ruling bloc has pledged to relocate the U.S. Marine Corps Air Station Futenma from the densely populated city of Ginowan to the Henoko coastal area of Nago, opposition-backed Denny Tamaki, who is against the move, secured a second term as Okinawa governor.

In April 2012, Ishihara, Japan's foremost nationalist who died earlier this year at the age of 89, unveiled his controversial plan to purchase the Senkakus during his speech at the Heritage Foundation, a Washington-based conservative think tank.

By speaking in the U.S. capital, Ishihara, long known as a critic of Japan-U.S. relations, tried to urge nations squaring off against China to bolster ties with Washington to ensure maritime security in the Asia-Pacific region, a source familiar with his thinking said.

Two years before the novelist-turned-politician, who called Japan "America's mistress," made the speech, a Chinese trawler collided with two Japanese patrol ships near the islets in 2010, fanning concern about a military conflict in the East China Sea.

China, meanwhile, overtook Japan as the world's second-biggest economy that year, prodding Ishihara to believe it became impossible to challenge China's military buildup without U.S. support, said the source who worked for the Tokyo metropolitan government.

Ishihara served as Tokyo governor for 13 years from 1999 after nearly three decades in national politics.

"Before he became Tokyo governor, Ishihara, an ocean lover, was earnestly interested in the situation surrounding the Senkaku Islands and highly vigilant against China's military expansion," the former municipal government official said.

During his first campaign for the gubernatorial race, Ishihara lambasted Walter Mondale, a former U.S. ambassador to Japan, for saying that Washington would not be compelled by the 1960 treaty to intervene in a dispute over the Senkakus.

Mondale was the U.S. ambassador to Japan for three years through 1996 under the administration of then President Bill Clinton.

"Despite China's rising assertiveness in the region, the United States may not have backed Japan even in case of a contingency in the East Chain Sea, while Japanese citizens lacked a sense of crisis. Ishihara was irritated," the former official said.

In the wake of Ishihara's surprise speech, the Japanese government began to strongly emphasize the significance of its security alliance with the United States, and public awareness of national security against China has clearly increased in Japan.

Recently, the United States and Japan, along with Australia and India, have been keen to strengthen cooperation with Indo-Pacific democracies. The four countries have formed the "Quad" partnership to counter China's growing clout in the region.

The tabloid of the Chinese Communist Party, The Global Times, has rapped the United States and its security allies for striving to build an "anti-China alliance in Asia" and establish an Asian version of the North Atlantic Treaty Organization.

In Japan, the ruling Liberal Democratic Party, headed by Prime Minister Fumio Kishida, has set a goal of doubling the nation's defense spending to 2 percent or more of gross domestic product in the next five years as part of efforts to tackle Chinese threats.

"Ishihara would have been satisfied, as his speech might have motivated the United States and the Japanese public to confront China," the former official added.

Critics, however, shrug off the view that the move by Ishihara, which eventually put pressure on Noda to nationalize the Senkakus, was irresponsible and triggered a sharp deterioration of ties between Japan and China.

For years after the nationalization, Japanese living in China had been forced to refrain from speaking their language in public due to intensified anti-Japan sentiment, said Masaru Kaneko, a 56-year-old Japanese businessman in Beijing.

Sino-Japanese relations "nosedived" after the nationalization and "have never really recovered despite some fluctuations," said Jeff Kingston, director of Asian Studies at Temple University Japan.

To achieve peace and stability in the region, Japan should "seek deeper diplomatic dialogue and greater engagement" with China to expand common ground on the occasion of the 50th anniversary of the normalization of diplomatic ties, he added.

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