Three of seven artificial islands in the Spratlys are designed as military bases, the American military says. Among them, Subi Reef has a harbor bigger than Pearl Harbor, and another, Mischief Reef, has a land perimeter nearly the size of the District of Columbia’s, a submarine warfare officer in the United States Navy, Thomas Shugart, said in a paper issued this past week.
Together, the three islands could probably accommodate as many as 17,000 military personnel and support aircraft able to deter or counter an American military intervention, said Mr. Shugart, who is serving as a senior military fellow at the Center for a New American Security in Washington but writes as an independent analyst.
Scarborough Shoal, with a deep lagoon covering nearly 60 square miles, offers an even bigger prize as a potential Chinese military base. “The picture would become even worse were China to build and militarize a similar island base at Scarborough Shoal,” Mr. Shugart wrote.
Both China and the Philippines claim the shoal, which the United States used as a firing range during the Vietnam War. Until 2012, Chinese and Filipino fishermen operated there. Then China seized the shoal, and Chinese Coast Guard vessels have chased away Filipino fishermen ever since.
Just 150 miles from the Philippine coast and Subic Bay, where the United States stations fighter jets and naval vessels, the shoal is in a particularly strategic place.
Its conversion into a military base would enable China to project military power across the South China Sea from a triangle of bases formed by the shoal, the Spratly archipelago to its south and the Paracel Islands farther to the west and closer to the Chinese mainland, Mr. Shugart said.
Anticipation regarding China’s plans for Scarborough has been building since March, when, at a meeting in Washington, President Obama warned President Xi Jinping of China against taking action that could activate American treaty obligations to the Philippines, a senior State Department official said.
In the heated arena of South China Sea politics in China, the shoal — known as Huangyan Island here — has become a touchstone for both hawks and more moderate voices.
Two fiery speeches at campuses in southern China in July by a popular current affairs television personality, Jin Canrong of Renmin University in Beijing, drew support from online chat forums. He said that China would begin construction on Scarborough Shoal next year, and that despite the warnings from America, “the spirit of President Xi is, ‘We must do it.’”
Mr. Jin described piling sand on coral at Scarborough as an ambitious project that would take four years but said that China needed to complete it to achieve de facto military control over the South China Sea.
The government’s enthusiasm for building on Scarborough Shoal was emboldened, he said, by the ease with which the three big artificial islands in the Spratlys were completed.
He quoted the commander of the Chinese Navy, Adm. Wu Shengli, as saying in a speech to his naval colleagues, “We didn’t expect President Xi would give us such robust support, we didn’t expect our engineering capacity would be so strong, and we didn’t expect the Americans would be so slow to react.”
Admiral Wu reinforced those views publicly at a meeting with the United States chief of naval operations, Adm. John W. Richardson, just days after the Hague decision. He said the South China Sea was a “core interest” that centered on the “foundation of the party’s governance, the country’s security and stability, and the Chinese nation’s basic interests.”
In an interview after his speeches, Mr. Jin said the tribunal ruling would have no effect on China’s plans for Scarborough Shoal. “The dominant perception of the political elite here is that the ruling is by five stupid guys,” he said, referring to the five lawyers who sat on the tribunal panel.
Still, Mr. Duterte’s openness to talking with China, and his cantankerous attitude toward the Americans, would probably delay the construction plans for Scarborough, Mr. Jin said.
A pause, he said, would allow for talks between China and Southeast Asian nations on a so-called Code of Conduct to lay down rules of behavior in the South China Sea. Premier Li Keqiang of China called for the code’s completion at a recent summit meeting of Southeast Asian nations in Laos.
Discussions of the code, under which some neighboring nations suspect China wants rules that would give it formal control of the South China Sea, have made little progress in nearly a decade, diplomats involved in the process say.
On the heels of Mr. Duterte’s election, China and the Philippines began preparatory talks last month at a meeting in Hong Kong between the former Philippine president Fidel V. Ramos and Wu Shicun, the president of the National Institute for South China Sea Studies, a Chinese government think tank.
The spokesman for China’s Foreign Ministry, Lu Kang, said Friday that China looked forward to Mr. Duterte’s visiting Beijing soon.
No matter what comes of discussions between China and the Philippines, the long-term objective of control of the South China Sea remains, said Shi Yinhong, a professor of international relations at Renmin University. “There just won’t be a straight-line approach,” he said.
And even though Mr. Duterte has spoken so brazenly about the United States — calling for the removal of American special forces from the restive southern part of his country and using vulgar language about Mr. Obama — China may not find it so easy to gain his confidence, Mr. Shi said.
Persuading the Philippines to abandon its ties to Washington and side with China, the way that two smaller Southeast Asian countries, Laos and Cambodia, consistently do, is unrealistic, he said.
The more realistic hope is that Mr. Duterte can be moved away from the United States just a bit, Professor Zhang, of Lingnan University, said. “That would be good enough,” he said.
But that would not necessarily resolve the future of Scarborough Shoal, he said, because China would never want to yield what it considers its sovereign right to the rocks so close to the Philippine coast.