Second Thomas Shoal (STS) has quietly become one of the most contested flashpoints in the South China Sea (SCS). In 1999, the Philippine Navy deliberately grounded the BRP Sierra Madre—a rusting World War II–era landing ship—on the reef to anchor Manila’s claim, and a small detachment of marines has lived aboard ever since.¹ To keep them there, the Philippines runs regular “rotation and resupply” (RORE) missions with food, water, and personnel. Since 2023, those missions have been met by increasingly aggressive Chinese Coast Guard (CCG) and maritime militia operations.²

At the same time, Washington and Manila have moved to clarify and revitalize their alliance: U.S. officials have explicitly stated that the 1951 Mutual Defense Treaty (MDT) applies to attacks on Philippine public vessels,³ including Coast Guard ships, “anywhere in the South China Sea.”⁴ President Ferdinand Marcos Jr. has drawn his own red lines around Filipino casualties and any attempt to damage or tow away the Sierra Madre.⁵ Marcos’ redlines clarified the scope of U.S.-Philippines MDT commitments and emboldened Philippine messaging; however, they have not resulted in China’s diplomatic concession or stopped Chinese coercion at STS. Instead, these signals have pushed Chinese maritime behaviors into a more dangerous gray zone where Chinese ships are willing to ram and board while staying right below the threshold of an “armed attack” that could trigger U.S. intervention. The analysis that follows examines how this dynamic has played out since 2023, revealing STS as both a critical stress test for the U.S.–Philippines alliance and, more significantly, a laboratory for China’s calibrated maritime coercion.

1. Sierra Madre: A Rusting Outpost

STS is a submerged reef in the Spratly Islands, lying within the Philippine exclusive economic zone (EEZ) but claimed by China as part of its Nansha Qundao.⁵ The Philippines asserts sovereign rights to the shoal and justifies its military outpost—the Sierra Madre—as a response to Chinese encroachment.⁶ China, on the other hand, maintains that STS is Chinese territory and denounces the grounding as an illegal occupation.⁷ The practical consequence is clear: both sides inhabit fundamentally different strategic realities at this single reef. Manila sees STS as an EEZ feature it must defend; Beijing sees any Philippine presence as an illegal occupation of Chinese territory. This perceptual gulf is what transforms routine resupply runs into confrontations where neither side can afford to back down.

The strategic logic behind the grounding was simple: as long as Filipino troops inhabit the Sierra Madre, China cannot occupy STS without directly attacking a Philippine public vessel and personnel. Any attack would be a dramatic and perilous escalation. Over two decades, this policy has become a symbol of Philippine resolve that frustrates China’s control of the area. At the same time, the ship’s deteriorating condition reveals a deeper strategic vulnerability for the Philippines—an aging vessel that becomes harder to resupply and defend with each passing month, yet one Manila cannot afford to abandon.

Chinese officials have been trying to flip the Philippines’ narrative by weaponizing disinformation. They frequently claim that Manila promised to remove the stranded ship in 1999 but never fulfilled that pledge—which Philippine officials publicly deny. As a Chinese foreign ministry spokesperson protested, 24 years later, the Philippines “not only failed to tow away that warship, but is now attempting major repairs and reinforcement to achieve permanent occupation of Ren’ai Reef.”⁸ Beijing argues that such actions violate international law and insists the Philippines must remove the ship and restore the shoal to an unoccupied status.⁹

Since 2013, China has maintained a regular Coast Guard patrol around STS.¹⁰ Under Philippine President Rodrigo Duterte (2016–2022), tensions over the shoal temporarily “cooled.” That cooling reflected a deliberate choice: Duterte downplayed the dispute and pursued closer ties and economic relationships with Beijing.¹¹ His posture shifted sharply when Ferdinand “Bongbong” Marcos Jr. took office in mid-2022. In essence, the Philippine administration “returned” to a pro-U.S. stance and adopted a more assertive policy in the SCS, renewing implementation of the Enhanced Defense Cooperation Agreement (EDCA) to host more U.S. forces and more openly challenging Chinese actions at sea.¹² Crucially, the U.S.–Philippines MDT was clarified in scope, with Washington reaffirming that the 1951 treaty applies to any armed attack on Philippine forces anywhere in the SCS.¹³ Beijing viewed this realignment with alarm. What had been a containable bilateral issue under Duterte now threatened to become a central arena in U.S.–China competition—precisely the outcome Beijing had worked to avoid. Collectively, these developments elevated STS from a dispute China thought it had effectively neutralized into something far more consequential—a test case for whether U.S. security commitments in the region held real weight.

2. Escalation in the Shoal

Starting in late 2022 and accelerating in 2023–24, every RORE mission became a potential flashpoint.¹⁴ China’s Coast Guard and maritime militia ramped up interceptions of Philippine supply boats. Tactics evolved: blockades, water-cannon attacks, physical ramming, and seizure of supplies became routine.¹⁵ A particularly intense period followed an August 2023 incident in which CCG ships blasted Philippine boats with military-grade lasers and water cannons, temporarily blinding crew members.¹⁶ Manila lodged formal protests, and the United States quickly issued a statement reiterating that any armed attack on Philippine public vessels in the SCS would trigger MDT obligations.¹⁷ By contrast, China defended the Coast Guard’s actions as “professional and restrained, beyond reproach,” insisting that the Philippines had “illegally transported construction materials” to the shoal and forced China to step in.¹⁸

According to data collected by the Asia Maritime Transparency Initiative (AMTI), the frequency of Chinese uses of force surged in late 2023 and included riskier maneuvers. One of the most alarming examples came on June 17, 2024, when Philippine resupply boats were aggressively rammed and besieged by CCG and militia vessels.¹⁹ AMTI analysts later noted that this June 17 clash was the closest yet to an “armed attack” on Philippine public vessels—the kind of incident that could have triggered U.S. intervention under the MDT.

Through late 2025, Chinese forces around STS have still not used lethal weaponry such as guns or missiles.²⁰ Coercive tactics have included water cannons, high-powered lasers, deliberate shadowing, ramming, and the deployment of small fast boats to intercept and seize supplies. By mid-2024, China began launching swarms of rigid-hull inflatable boats (RHIBs) from larger Coast Guard ships to chase down Philippine dinghies.²¹ From Beijing’s standpoint, the challenge is to assert its claim over STS and frustrate the Philippine outpost without triggering an incident that Manila (and Washington) could clearly define as an “armed attack” under the MDT.

3. Alliance Signaling and Philippine Red Lines

As Chinese harassment at STS escalated, the United States and the Philippines worked to send stronger deterrent signals against China. Long-standing ambiguity over the MDT’s geographic scope and application to maritime incidents was effectively removed. Top U.S. officials repeatedly clarified in 2023–2024 that the MDT covers the SCS and specifically includes Coast Guard or other public vessels—closing off any supposed legal loophole for gray-zone aggression at sea.^{22 23} After major collision incidents in late 2023, the U.S. State Department explicitly condemned China’s dangerous actions and reinforced the U.S.–Philippines 2+2 ministerial joint statement, which warned that any armed attack in the Pacific would lead to joint defense measures.²⁴ By broadcasting these commitments, Washington aimed to deter Beijing from escalating violence at STS while also reassuring Manila that it would not face Chinese coercion alone.

The Philippines, for its part, grew more confident in the strength of the alliance. For instance, Philippine officials publicly defined “red lines” that would justify a request for U.S. military help. In late August 2025, amid another confrontation, a Philippine Navy spokesman openly warned that any attempt to remove the Sierra Madre by force or to board it “will be seen as crossing a red line.”²⁵ Such firm public messaging from Manila was practically unheard of a few years ago.

From Beijing’s perspective, however, the Philippines’ emboldened rhetoric amounted to the United States “goading the Philippines into provoking China” and engaging in “hollow intimidation.”²⁶ Chinese officials and state media repeatedly accused Washington of being the “black hand” stirring trouble in the SCS and claimed that U.S. “instigation and support” emboldened the Philippines to provoke China.²⁷ Chinese media warns that aligning too closely with Washington would backfire on Manila.²⁸ The threat here operates on two levels, both designed to raise Manila’s costs. Economically, closer U.S. alignment invites Chinese trade retaliation or an infrastructure investment pullback—leverage Beijing has wielded before. Strategically, hosting more U.S. forces makes Philippine bases potential first targets in any Taiwan contingency, transforming the country from a peripheral actor into frontline real estate. What Beijing is really signaling, in my view, is that the MDT’s protection comes with an unacknowledged price: the Philippines absorbs disproportionate risk while serving as a forward operating base for U.S. strategic interests. That matters because Manila’s geographic vulnerability means it cannot dial down its exposure the way Washington can. Beijing has reinforced this framing publicly: Foreign Ministry spokesperson Mao Ning blasted cooperation on U.S. missile deployments as “a typical example of harming others and oneself.”²⁹

Furthermore, Chinese strategists describe the situation as the U.S. manipulating the Philippines to contain China.³⁰ A 2025 Chinese analysis argues that Washington has been leveraging the alliance by building new mini-lateral security networks—including with Japan and Australia—influencing SCS legal norms to disadvantage China, and upgrading military cooperation with Manila. This has all been framed as part of U.S. “great power competition” and efforts to encircle China along the First Island Chain.³¹ From China’s narrative, Second Thomas Shoal is not just about Manila and Beijing; it is a triangular testing ground for U.S.–China rivalry. Chinese commentators have portrayed the 2023–24 shoal standoffs as exactly that: the U.S. encouraging the Philippines to “test” Chinese resolve while avoiding direct involvement itself.³² This framing reveals something important about gray-zone dynamics: STS isn’t a level playing field where all parties test each other equally. Instead, it’s a laboratory where China steadily expands what counts as ‘normal’ maritime coercion while the U.S. and Philippines work primarily to manage escalation rather than reverse Chinese gains.

4. Beijing’s Logic & Response at Home: Firmness Without War

Beijing’s fundamental position on STS is driven by sovereignty and precedent. Allowing the Philippines to solidify control over Ren’ai Reef is seen as undermining China’s broader claim over the Spratlys and encouraging other neighbors (like Vietnam or Malaysia) to fortify their own occupied features. Chinese officials insist they will never allow permanent Philippine occupation of the shoal.³³ From this perspective, every Philippine resupply mission carrying construction materials—steel, cement, metal braces, and more—is a step toward “permanent occupation,”³⁴ which is explicitly deemed unacceptable in Chinese diplomatic statements.³⁵ Fear-driven internal logic justifies China’s on-scene tactics as necessary “law-enforcement” actions on Chinese territory. The CCG repeatedly claims that its measures at STS are “in accordance with law,” flipping the blame by accusing Philippine vessels of “dangerous provocations.”³⁶ Yet this maximalist approach traps Beijing in its own logic. If China is acting as a rational great power, waging repeated high-risk confrontations over resupply missions to a deteriorating ship seems like an unusually brittle way to defend sovereignty—especially when any misstep could spiral from controlled ‘law enforcement’ into an unintended crisis.

More broadly, Chinese government and media outlets have leaned on disinformation and selective readings of international law to defend expansive maritime claims,³⁷ arguing that China has “indisputable sovereignty” and “historic rights” in the SCS based on “long-standing practice.”³⁸ When criticized over its interpretation of the United Nations Convention on the Law of the Sea (UNCLOS) and challenged on the legality of its nine-dash line, China’s Foreign Ministry declared the 2016 arbitral award “null and void and without binding force,” saying China neither accepts nor recognizes it.^{39 40}

This combination of legal revisionism and outright rejection of arbitration serves a clear domestic purpose: maintaining Chinese Communist Party legitimacy, which rests heavily on a narrative of ending the ‘century of humiliation.’ The domestic constraint operates through an ironic logic—tight censorship intensifies the problem rather than solving it. Beijing has deliberately cultivated nationalist investment in maritime sovereignty through coordinated state media campaigns, educational curricula, and tightly managed social media discourse. The Party controls the narrative, which means it also owns the consequences. Once you’ve told political elites, military leadership, and an educated public that certain maritime claims represent core national interests and non-negotiable symbols of China’s return to great-power status, admitting legal defeat becomes politically hazardous precisely because those audiences have internalized the story. The real risk isn’t spontaneous grassroots protest—censorship can suppress that. It’s criticism from within the system itself: nationalist officers in the PLA who’ve been told these waters are Chinese, diplomats who’ve spent years defending the nine-dash line, provincial leaders steeped in the sovereignty narrative through Party education. Backing down on arbitration would undercut the Party’s own legitimacy story about having ‘stood up’ China—a vulnerability censorship tools can’t fully neutralize because the constraint comes from elite audiences the Party itself conditioned. From Beijing’s perspective, absorbing reputational costs abroad remains safer than appearing weak at home on an issue it has explicitly elevated to core-interest status.

China has also shown considerable flexibility at the tactical level. After the near-crisis of June 2024, Chinese and Philippine officials reportedly reached a tentative understanding to de-escalate at STS. According to Chinese statements, this “new model” involved the Philippines agreeing not to send large-scale construction materials and perhaps to provide notice of resupply missions. In late July of 2024, a resupply mission was allowed through, nonetheless, without triggering an incident.⁴¹ Chinese strategists likely realized that a short tactical relaxation could deflect U.S. and international criticism while preserving Beijing’s long-term goal of eventually ousting the Sierra Madre on more favorable terms.

5. Uncertainties at Second Thomas Shoal: A Dangerous New Normal

Second Thomas Shoal is, on the map, just a tiny reef. But it has practically become a stress test for the U.S.–Philippines alliance and a ramp for China’s calibrated coercion in SCS. The alliance clarifications did not persuade Beijing to back off; if anything, they hardened current positions and led to standoffs.⁴² Manila is increasingly more determined to hold the line at the Sierra Madre and more willing to call out Chinese coercion, largely because of its confidence in its alliance with the U.S. Beijing, convinced that the U.S. is “using” the Philippines to contain China, has focused on using gray-zone tactics to eventually dislodge the outpost without triggering an actual war.

The problem is that this “managed” confrontation is inherently unstable. Each RORE mission is now subject to potential miscalculation, with Chinese and Philippine personnel literally colliding at sea. Another bad day at the shoal could prompt Manila to invoke MDT and put Washington on the spot. In that sense, Second Thomas Shoal is more than a distant reef; it is where China is learning how far gray-zone coercion can go under a clarified U.S.–Philippines alliance.

Work Cited

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42. 海陆空武器装备, “仁爱礁成战略测试场.”