The conventional wisdom in Washington these days is that if the United States were to ever go to war with China, it would likely be over Taiwan.

But there’s another scenario that is at least just as likely.

The Philippines and China are locked in a long-running dispute over a shallow water reef in the South China Sea known as the Second Thomas Shoal, which is only visible during low tide.

“The Second Thomas Shoal off the coast of the Philippines could be the next flashpoint to launch World War III,” Rep. Mike Waltz (R-FL), a former Green Beret, said in April. “Our enemies and our adversaries no longer respect and fear us, and that has happened and deteriorated precipitously in the last three years.”

The Second Thomas Shoal is just one of more than 100 submerged features — shoals and reefs — which do not qualify as islands under international law, but to which China asserts “historic rights,” as the basis for laying claim to 90% of the strategic South China Sea, including the fishing rights and its significant undersea oil and gas deposits.

It’s a claim, the U.S. insists, that “has no legal basis.” 

“The PRC [People’s Republic of China] asserts claims to internal waters, a territorial sea, an exclusive economic zone, and a continental shelf that are based on treating each claimed South China Sea island group ‘as a whole,’” the State Department said in a 2022 report. “This is not permitted by international law.”

The Philippines is one of several Asia-Pacific nations — including Brunei, Indonesia, Malaysia, Singapore, Taiwan, and Vietnam — that claims the waters nearest to its coast, which it calls the West Philippine Sea, as its territory, rejecting China’s declaration of sovereignty over almost all of the South China Sea. 

Of seven nations, the Philippines has been the most aggressive in challenging Beijing’s expansive claims for more than two decades.

In 1999, the Philippines beached an obsolete World War II-era landing ship, the BRP Sierra Madre, on Second Thomas Shoal, and manned it with sailors from the Philippine navy as a tangible way to exert its sovereignty over the 11-mile-long partially submerged reef. 

Then in 2016, the Philippines took its case to the Permanent Court of Arbitration at The Hague, which ruled that China had no legitimate claim to the shoal under a provision of the U.N. Convention on the Law of the Sea, to which both countries are a party. 

But now, after 25 years of being subjected to the elements, the BRP Sierra Madre is an unseaworthy, rusting hulk, and China is doing everything short of opening fire on the ramshackle ship to try to keep the Philippines from resupplying the small crew holding down the fort on the Second Thomas Shoal, which they call the Ayungin Shoal, in defiance of Beijing.

“PRC ships employed dangerous maneuvers and water cannons against Philippine vessels carrying provisions to Filipino service members stationed at the BRP Sierra Madre, causing multiple collisions, damaging at least one Philippine vessel, injuring Filipino service members, and jeopardizing the safety of the Filipino crew,” the State Department said in a statement in March. “We condemn the PRC’s repeated obstruction of Philippine vessels’ exercise of high seas freedom of navigation and its disruption of supply lines to this longstanding outpost.”

The U.S. has a policy of not taking sides in the territorial disputes, while asserting the absolute right to sail and fly over what it insists is international airspace and waters. 

But in a speech last June, Defense Secretary Lloyd Austin made clear that the U.S. was backing Manila over Beijing. 

“Let me again underscore the importance of the 2016 ruling by the arbitral tribunal. It is legally binding, and it is final,” Austin said at the annual Shangri-La Dialogue in Singapore.

The battle for the submerged spit of sand and rock is growing more contentious and more dangerous, with China using its numerically superior navy along with civilian fishing vessels to blockade the shoal.

If China were to move against Taiwan, the president and Congress would have tough decisions to make about how far the U.S. would go to defend the self-governing island.

Would the U.S. be limited to weapons and ammunition, as is the case in Ukraine? Or would the U.S. military intervene with troops, ships, and planes in an all-out war?

But in the case of the Philippines, there is no question that, under the terms of a 1951 mutual defense treaty, the U.S. is obligated to come to Manila’s defense.

During a visit to Washington last month by Philippine President Ferdinand Marcos Jr., President Joe Biden “reinforced the ironclad U.S. alliance commitment” to its treaty obligations, according to a White House statement.

At the Pentagon the next day, Austin echoed the promise.

“Our commitment to the defense of the Philippines is ironclad,” Austin said, standing next to Marcos. “An armed attack on Philippine Armed Forces’ public vessels or aircraft in the Pacific, including the South China Sea, would invoke U.S. defense commitments under our mutual defense treaty.”

The words are intended to deter China from miscalculating, but Beijing continues to ramp up confrontations with Philippine resupply vessels, swarming the ships and dousing them with high-pressure water cannons.

“The kind of behavior that we’ve seen where Filipino crews are put in danger where, you know, sailors have been injured and [there has been] property damage, that’s irresponsible behavior and it disregards international law,” Austin said at a gathering of Pacific allies in Hawaii this month.

Asked by a reporter if the death of a Filipino sailor would trigger the mutual defense pact, Philippines Defense Secretary Gilberto Teodoro replied that it would be “counterproductive to delve into hypotheticals.”

“We are very conscious of the fact that we need to assert our rights but in a manner that safeguards the safety of each and every member of the Philippine Armed Forces,” Teodoro said, adding that “other countries” sometimes overstate the threat of U.S. intervention as a “bogeyman” to complain about legitimate defensive measures.

“It is an agreement and it will be a political decision at the end of the day of principally the Philippine government when to invoke it,” he said.

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“If as our allies, the Philippines, were to be attacked, then that would change a bit of the scope of what’s happening there at Second Thomas Shoal,” retiring Adm. John Aquilino, the outgoing Indo-Pacific commander, told the Senate Armed Services Committee in March.

“I believe there’s a peaceful way out of this. There’s a place for China in this world,” Aquilino testified. “They’re just going to have to understand that the nations of the globe require a set of international standards and behaviors that are acceptable and what they’re executing now is not.”

Jamie McIntyre is the Washington Examiner’s senior writer on defense and national security. His morning newsletter “Jamie McIntyre’s Daily on Defense,” is free and available by email subscription at dailyondefense.com.