SINGAPORE—A dispute between China and the Philippines, a U.S. ally, is rapidly escalating over an unusual military outpost: a World War-II era ship that is leaky, riddled with holes, covered in rust and sitting atop a reef in the South China Sea.
The decrepit ship, the BRP Sierra Madre, and the small detachment of marines aboard are defending the Philippines’ claim to Second Thomas Shoal, located about 100 miles off its west coast. The country grounded the ship on the reef 2½ decades ago to stave off China’s expanding control over the South China Sea.
The Philippines sends regular supplies to the marines on the ship, and China is trying to make those missions as difficult as possible. The Chinese coast guard has aimed a military-grade laser and shot a water cannon at Philippine vessels. On Sunday, a Chinese coast guard ship and maritime militia boat collided with Philippine vessels that were headed to Second Thomas Shoal.
The stakes are high, not just for the Philippines but also for the U.S. If tensions continue to rise, they could draw in Washington, which has a security alliance with Manila rooted in a decades-old mutual-defense treaty.
So far, the U.S. has shown strong support for its Southeast Asian ally and sought to deter further Chinese escalation. Navy destroyer USS Dewey was over the horizon from the site of Sunday’s encounters between Chinese and Philippine ships—present, though keeping its distance. An American patrol aircraft was at the scene during a resupply run last month, a Philippine armed-forces spokesman said.
U.S. national security adviser Jake Sullivan spoke to his Philippine counterpart Monday, and said the U.S.’s alliance commitments to the Philippines were ironclad. The State Department called China’s actions dangerous and unlawful, as did other U.S. officials in separate comments. A spokeswoman for the U.S. Seventh Fleet declined to comment on specific military operations but said all of its activities were done in coordination with the Philippines.
The tensions in the region have been rising as Manila contends with a pressing problem: The Sierra Madre is in bad shape after being exposed to the elements for nearly 25 years. The Philippines has carried out repairs incrementally over time, but the ship won’t stay intact forever—and some think its days are numbered.
“You can only repair it so much,” said Adm. Rommel Ong, who retired as a vice commander of the Philippine navy in 2019. “At some point, it will still degrade.”
If the ship starts to fall apart, Chinese forces would likely mobilize to take control of Second Thomas Shoal, squeezing out the Philippines. Beijing might be accelerating the ship’s decline by trying to block Manila from carrying out even patchwork repairs, said Jay Batongbacal, a maritime-affairs expert at the University of the Philippines College of Law.
Beijing objects to repairs on the Sierra Madre, which it has asked the Philippines to tow away. It has said it is intercepting boats because the Philippines is trying to transport construction materials alongside food and other necessities. The Philippines says it has a duty to support its marines on the ship.
“We have to make sure that vessel remains livable for our troops that are stationed there,” said Col. Medel Aguilar, spokesman for the Philippine armed forces. “We are doing something that will make sure it remains there and remains livable.”
Any move by China to seize Second Thomas Shoal would put pressure on the U.S. to back up its ally—not just diplomatically but also militarily. That could bring the U.S. into direct conflict with China, carrying huge risks for both powers. Beijing might see a window of opportunity while the U.S. is preoccupied with the wars in Ukraine and the Middle East, security analysts said.
The U.S. response to developments in the South China Sea are under greater scrutiny because of events in 2012 that scarred the alliance and caused lasting national trauma in the Philippines. That year, after a weekslong standoff between Chinese and Philippine forces, China seized a coral atoll called Scarborough Shoal. Officials in Manila said later the U.S. hadn’t done enough to support its ally, allowing Beijing to wrest control with few repercussions.
If China succeeded similarly at Second Thomas Shoal, it would send “very bad signals about the U.S. alliance and the effectiveness of its security guarantees,” said Batongbacal.
“That’s why I think it’s such a tempting target for China,” he said. “If they’re able to swing this, they will be able to basically crack a U.S. alliance in the region and the credibility of U.S. security guarantees.”
China’s tactics around Second Thomas Shoal have grown more aggressive this year.
In February, the Philippines said its coast guard crew was temporarily blinded by the military-grade laser flashed by China’s coast guard. A few months later, it was a water cannon, slamming into a Philippine boat. In early October, a Chinese coast guard ship came so close that the Philippines said it had to throw a vessel into reverse to avert a collision. This weekend, collisions occurred—not just one, but two.
The confrontations have come as the Philippines has shifted its stance on its neighbor. Since coming to power last year, President Ferdinand Marcos Jr. has reversed the pro-China policies of his predecessor. He has doubled down on the alliance with the U.S. and pushed back against Beijing in the South China Sea. Philippine coast guard and armed forces frequently call out Chinese actions, putting out photos and videos showing its tactics.
“It is no longer as easy for China to get away with what it is doing in the South China Sea,” said Collin Koh, a senior fellow at the S. Rajaratnam School of International Studies in Singapore. As the Philippines has fought back by broadcasting encounters between the two countries, China has “found itself being humbled or humiliated by what was going on,” he said.
China might also have accelerated its actions to make sure the Philippines doesn’t go any further to cement its presence on Second Thomas Shoal, such as by building permanent structures there, Koh said.
Some security analysts say the U.S. should do more to assist the Philippines. The U.S. Navy could deploy ships to escort Philippine resupply convoys to Second Thomas Shoal or even go a step further by helping to protect the Philippines if it tries to erect structures there, they say.
These are tricky calls, said Koh, adding, “It’s a game of chicken.”
Write to Niharika Mandhana at niharika.mandhana@wsj.com