When a U.S. guided missile destroyer sailed within 12 nautical miles of a Chinese-claimed island in the disputed South China Sea last month, it set off a volley of protests from Beijing.

“The U.S. is challenging and provoking the new maritime order by wielding its military power,” Foreign Ministry spokesman Lu Kang said. China’s defense ministry said it would bolster its capabilities in the area as needed.

The resumption of U.S. “freedom of navigation operations” in the South China Sea — with three since October after a three-year hiatus — reflects the global focus on a maritime route that carries $5.3 trillion of global trade a year. Tensionsbetween China and Southeast Asian nations over the waters sit at the center of a rivalry between the U.S., overseer of the region’s security network for decades, and a rising China intent on becoming the region’s dominant power.

“The South China Sea has become one of the main sticking points in Sino-U.S. relations, which used to be more troubled by trade and currency,” said Zhou Qi, director of the National Strategy Institute at Tsinghua University. “As the U.S. increasingly pivots to the Asia-Pacific, boosting partnerships with old allies and building new friendships — many of which have territorial disputes with China — Beijing feels the pressure.”

After years of simmering friction, the disputes have taken on some urgency as an international arbitration court in The Hague prepares to rule on a case brought by the Philippines against China. A ruling seen as unfavorable to Beijing could undermine its claims to more than 80 percent of the waters.

The Philippines has asked the court to rule on the status of features China claims as well as the legal basis of its “historic rights” claim, based on a 1940s map showing a vague dashed line looping down about 1,119 miles south of Hainan island and covering around 1.4 million square miles. The area overlaps claims from Vietnam, Malaysia, the Philippines, Brunei and Taiwan.

China — which has declined to participate in the case — and the U.S. have embarked on a diplomatic and public relations frenzy before the ruling, expected within months. The U.S. is not a claimant but has championed free navigation in the area.

The Group of Seven nations expressed concern about instability in the South China Sea at a meeting in Japan last month.

For its part, China claims the support of countries as varied as Russia, Gambia, Belarus and Kazakhstan. In recent weeks, more than a dozen Chinese ambassadors — from the U.K. to Sierra Leone — have published articles backing the country’s stance, and this week a paid supplement arguing the case was published in the Jakarta Post. The same day the Manila Times published an article by Fu Ying, chair of the foreign affairs committee of China’s National People’s Congress, that accused the Philippines of “hyping” up the dispute.

“China is going to lose and that will be humiliating for them, so they are putting forward an alternative argument which isn’t really well defined,” said Bill Hayton, author of “The South China Sea: The Struggle for Power in Asia.” Hayton reckons China will lose at least half of the 15 submissions the Philippines made to the court. “There is a massive effort to try and show that China has international support for its position.”

The U.S. inserted itself into what had been previously a mostly regional dispute when, in 2013, China started to dredge and dump millions of tonnes of sand and coral onto seven features in the Spratly chain, which lies in the southeast of the South China Sea. China has created artificial islands covering 3,200 acres of land — more than three times the size of Central Park in New York.

China’s foreign ministry says the country has the right to build on the features because they are its ”indisputable” sovereign territory and, in any case, it’s mainly to provide civilian services like maritime search and rescue.

That isn’t how the U.S. views it. “We assess that China has established the necessary infrastructure to project military capabilities in the South China Sea beyond that which is required for point defense of its outposts,” James Clapper, the director of national intelligence, saidin March.

U.S. officials have said they are concerned China is taking actions that are too minor to prompt a response, but which over time equate to substantial change. They contend that China’s eventual aim is to establish forward military bases and push back the U.S. navy.

China argues it has not hindered commercial navigation. But when it seized the Scarborough Shoal from the Philippines in 2012, it restricted banana imports from the country, citing health reasons. In 2010, China effectively banned the export of raw earths to Japan amid tensions over disputed islands in the East China Sea.

China is increasingly relying on its coast guard and armed fishing boats to assert its claims. The coast guard though lies outside its navy, which has an agreed code with the U.S. for unplanned encounters at sea, and with whom it has occasionally taken part in multilateral exercises.

“In turning away from tactical aggression, Beijing has refocused on passive assertive actions to consolidate a new status quo in maritime Asia,” wrote Ashley Townshend and Rory Medcalf in an April report by the Sydney-based Lowy Institute for International Policy.

They said that because it is now virtually impossible to compel China to roll back its outposts, the priority for the U.S. and its partners should be to “deter further militarization.”