Japan is shaping up as China’s chief rival in the disputed South China Sea because it has a sustained, multi-pronged approach and a unique set of reasons to test Beijing’s growing influence, analysts say.The country has emerged since 2017 as a force in the sea. It works with the United States on joint naval exercises including one from May 2-8 that also involved warships from India and the Philippines. Japan has separately sent its Izumo-class helicopter carrier to the sea at least four times since 2017. That year it took a three-month tour.Japan, unlike the United States, is helping Southeast Asian claimants to the South China Sea develop infrastructure and maritime firepower. Japanese officials don’t claim the sea, but a separate territorial dispute with China motivates them to keep a watch on Chinese influence. Japanese citizens tend to support foreign policy targeting China, analysts say.“Under (Japanese Prime Minister Shinzo) Abe, Tokyo has enhanced defense and security engagements in Southeast Asia, not least with the South China Sea in mind,” said Collin Koh, maritime security research fellow at Nanyang Technological University in Singapore. “I believe Beijing will...be concerned about Japan using the South China Sea.”US allyAt least six other countries with no claims in the South China Sea sovereignty dispute have sent ships into the waterway over the past three years. They are Australia, France, India, Singapore, the United Kingdom and the United States. All but Washington send ships too sporadically to cause major alarm in China, despite protests from Beijing’s foreign ministry, scholars have said. The United States operates the world’s strongest armed forces, and the U.S. Navy has sailed in the sea 11 times under President Donald Trump alone. China worries most about the United States, followed by Japan, Asia scholars say. That's partly because Japan is a U.S. treaty ally of nearly 60 years.“Japan is more northeast focused, but it does venture down to the South China Sea,” said Carl Thayer, emeritus professor at the University of New South Wales in Australia. “You can’t push Japan around without thinking about what the U.S. would do.”Public sentimentAlso setting Japan apart, its government has sparred with China since 1996 over sovereignty in parts of the East China Sea. Most Japanese fret over China’s rise, giving their government a stronger mandate to check Chinese expansion through alliances with other countries, said Jeffrey Kingston, history instructor at Temple University’s Japan campus.“I think there’s a lot of concern about rising China and about North Korea, so in this environment I think the government, which in a way helps to stoke those anxieties, is seizing the opportunity to build these coalitions (and) partnerships,” Kingston said.Persistence, economic aidThe frequency and duration of Japan’s Izumo tours of the 3.5 million-square-kilometer South China Sea stand out from other outside parties except for the United States, analysts say. After the May 2-8 exercises, the Izumo joined Australian, French and U.S. ships west of the Indonesian island of Sumatra for another round of South China Sea exercises.Brunei, Malaysia, the Philippines, Taiwan and Vietnam claim all or parts of the sea, which stretches from Hong Kong to Borneo. Their exclusive economic zones all overlap the 90 percent that China calls its own. Militarily weaker than Beijing, the Southeast Asian claimants resent China’s use of reclaimed land to build up tiny islets for aircraft hangars, radar systems and support for oil exploration. Claimant countries prize the sea as well for its marine shipping lanes and undersea reserves of gas and oil. Japan relies on it for raw materials and shipping routes for its exports. Unlike other outsiders to the dispute, Japan has offered direct military support to countries that contest Chinese maritime claims. Japan's Self-Defense Forces agreed last year to join the United States in joint exercises with the Philippines. This month the Japanese Defense Minister met his Vietnamese counterpart to tighten their maritime security ties.Japanese officials are lending money to both Southeast Asian countries for infrastructure projects.Chinese reactionChina cites documents going back to dynastic years to show historic use of the sea, bolstering its claim. The Chinese foreign ministry protests when U.S. ships or U.S.-allied ships sail there, calling the moves outside intervention in a regional dispute.Japan’s activity is unlikely to stop China from operating in the sea’s Paracel Islands, a chain it has controlled since the 1970s, or militarizing three major islets in the Spratly archipelago, scholars have said. But they say the activity may help deter China from taking more islets claimed by other countries.“China will reverse the argument to say ‘you all are militarizing the South China Sea, not me, and I think they will just play that game and not do anything further,” said Alan Chong, associate professor at the S. Rajaratnam School of International Studies in Singapore.