An international tribunal in The Hague delivered a stinging rebuke to China last week, ruling that China's claims to nearly the entire South China Sea were invalid.
The decision also questioned the legality of China's claim of — and construction on — several reefs also claimed by the Philippines, which brought the case. China says it won't abide by the ruling. And some in the Philippines worry China will go ahead with building activity on Scarborough Shoal, a section of rocks and reef which it seized in 2012. The shoal sits just 110 nautical miles from the main Philippines island of Luzon.
"Every reef they've seized they've made into an island," says Antonio Carpio, a senior associate justice of the Philippines Supreme Court. "What makes Scarborough Shoal exceptional? Nothing."
Carpio is a vocal defender of the Philippines' territorial claims in its dispute with China. He says a Chinese presence on Scarborough Shoal would threaten not only the Philippines, but also U.S. forces using Philippine bases under a new, enhanced defense cooperation agreement.
"If you have an airfield there, maybe it will take just 15 minutes for the fighter jets there to reach Manila," he says. "And the U.S. forces using Clark [Air Base] and Subic [naval base] are all within range."
That fact is not lost on the United States. The U.S. has consistently said it has no dog in the fight over conflicting claims in the South China Sea. But in recent months the U.S. has conducted a series of high-profile freedom of navigation operations in the disputed waters, near the artificial islands China has created there.
In late June, two U.S. aircraft carrier strike groups conducted joint operations in the Philippine Sea ahead of the tribunal's decision. And U.S. warplanes based at Clark Air Base conducted patrols near Scarborough Shoal.
"Definitely the U.S. has sent some strong signals to the Chinese that they're willing to do more than they're used to," says Jay Batongbacal, director of the Institute for Maritime Affairs and Law of the Sea at the University of the Philippines.
Batongbacal thinks the increased U.S. presence and the ruling of the court may force China to hit the pause button.
"Even their strategists would know, I think, that the Scarborough Shoal would be a tipping point for the U.S. and Japan, given how the situation has radically changed," he says. "Because it would complete the so-called strategic triangle that could finally establish full control over the South China Sea, and they would know that the U.S. and Japan will not allow that to happen easily."
But does the Scarborough Shoal really represent a red line for the U.S. — one worth the risk of open conflict with China?
Richard Heydarian of Manila's De La Salle University isn't so sure. He's the author of Asia's New Battlefield: The USA, China and the struggle for the Western Pacific.
"We already heard this red line statement on Syria, and clearly saw how [it was] not [a] red line after all," he says. He says many Filipinos, including the new president Rodrigo Duterte, fear the same "artificial posturing red line" on the Scarborough Shoal.
Heydarian says that mistrust of U.S. support helps explain the Philippines' tempered response to the court's verdict.
In return for "the Philippines not flaunting and taunting the verdict," he speculates, "China will give guarantees in the short term at least that it will not up the ante, it will not establish facilities in the Scarborough Shoal and will actually perhaps give Filipino fishermen more access to that area."
That hasn't happened so far. Filipino fishermen who tried this week were again turned back by Chinese vessels.
But China and the Philippines have been cautious — at least with each other — in their reaction to the tribunal's ruling. There's an expectation here that this restraint will last, at least for a few months. The softer approach, adopted by new Filipino President Rodrigo Duterte, runs in stark contrast to the rancor that characterized relations between China and the Philippines under his predecessor, Benigno Aquino III.
Jay Batongbacal expects the two sides to sit down for bilateral talks on solving their dispute peacefully.
"As long as they don't make the situation any worse by taking an even harder line," he says, or "additional unilateral action, I think there will be some room, at least, for both parties to step back from the collision course that they seemed to be on and work out a mutually acceptable solution."
Carpio, the Supreme Court justice, agrees that the Philippines and China will likely sit down and talk, especially about exploiting natural resources beneath the sea. But he doesn't expect China to compromise on Scarborough Shoal. He expects China to fill it in and build, similar to what China did with with Spratly Islands further to the south. The Philippines can't stop it, Carpio says. It's up to the Americans.
"I don't know the answer to that, whether they can enforce that red line or not," Carpio says. "But they will lose a lot of credibility if they say there is a red line and the red line disappears."
He says it doesn't just matter to the Philippines. Japan, Vietnam and other countries engaged in maritime disputes with China will take note of what Washington does next.