US Invokes Mutual Defence Treaty After Chinese Coastguard Water Cannon Incidents Near Scarborough Shoal

The Water Cannon Incidents and Their Context

The 17 October water cannon incidents occurred during a scheduled joint US-Philippines freedom of navigation and resupply patrol near Scarborough Shoal (Bajo de Masinloc), a reef lying approximately 220 kilometres west of Luzon within the Philippines' 200-nautical-mile Exclusive Economic Zone. The patrol — which included two Philippine Navy vessels, two Philippine Coast Guard ships, and a US Navy P-8A Poseidon maritime patrol aircraft overhead — was attempting to approach the shoal for a scheduled resupply of Filipino fishermen who regularly operate in the area. Chinese Coast Guard vessel Jiaolong (hull number 5901) and two smaller CGC support vessels intercepted the patrol at approximately 12 nautical miles from the shoal and deployed water cannon against the Philippine vessels BRP Sierra Madre and BRP Bagacay, causing hull damage to the former and injuring two Philippine sailors.

The incidents are the latest in a series of escalating Chinese coastguard operations against Philippine vessels that have intensified since 2023. Previous incidents involved water cannon employment against resupply missions to the BRP Sierra Madre — a deliberately grounded vessel at Second Thomas Shoal that serves as a forward observation post — laser illumination of Philippine vessels, and physical blocking of maritime access. The 17 October incidents are distinctive in three respects: they occurred at Scarborough Shoal rather than the more contested Second Thomas Shoal; they targeted vessels on a joint US-Philippines patrol rather than purely Philippine national assets; and they caused physical injury to Philippine military personnel, crossing a threshold that previous incidents had not reached.

US MDT Invocation: Legal and Strategic Significance

The US State Department and Department of Defense's joint statement invoking the Mutual Defence Treaty was carefully worded to cover Philippine 'armed forces, public vessels or aircraft in the South China Sea' — broader language than previous US government statements that had stopped short of explicit application of the MDT to the South China Sea context. Previous administrations had maintained studied ambiguity about whether MDT obligations extended to Philippine South China Sea operations, consistent with the longstanding US position of not taking sides on underlying territorial sovereignty disputes while maintaining treaty obligations to Philippine armed forces. The 18 October statement resolves that ambiguity in the direction of broader coverage.

China's official response characterised the MDT statement as 'irresponsible interference in legitimate Chinese law enforcement operations' and reiterated Beijing's position that Scarborough Shoal is Chinese sovereign territory over which China exercises legitimate jurisdiction. The Chinese Foreign Ministry summoned the US chargé d'affaires and the Philippine ambassador for formal protests. Chinese state media framed the incident as evidence of US-Philippine 'military provocation' and warned that continued joint patrols would be met with 'resolute countermeasures.' The practical question is whether the MDT invocation will deter further Chinese coastguard escalation or whether Beijing will continue probing operations while calibrating actions to remain below what it assesses as the US threshold for direct military response.

Philippines' Strategic Positioning and Capability Development

The Philippines under President Ferdinand Marcos Jr. has pursued a more assertive posture in the South China Sea than his predecessor, including through dramatically expanded US basing access (nine Enhanced Defence Cooperation Agreement sites as of 2025), increased joint US-Philippines patrols, and public disclosure of Chinese coastguard incidents that was previously suppressed for diplomatic reasons. Manila has simultaneously pursued a coastguard fleet expansion with Japanese financing, acquiring Shirane-class vessels and smaller patrol craft that provide more capable assets for South China Sea operations than the ageing vessels that constituted most of the Philippine fleet two years ago. The combination of expanded US support and improved Philippine capability has raised the cost of Chinese coastguard harassment while also creating more opportunities for incidents.

The MDT invocation's long-term effect on South China Sea stability depends substantially on how other ASEAN claimant states — particularly Vietnam and Malaysia, which have their own South China Sea disputes with China — read the US commitment. Vietnam and Malaysia have historically avoided reliance on US security guarantees for their South China Sea claims, preferring a hedge that maintains economic relations with China. If the US MDT invocation creates a more credible deterrence framework around Philippine claims, it may embolden claimants to adopt more assertive postures that Beijing would then need to respond to across multiple fronts simultaneously — a dynamic that could either deter Chinese escalation or produce a broader South China Sea confrontation.

Sources

• US Department of State – Statement on US-Philippines Mutual Defence Treaty and South China Sea
• Philippines Department of Foreign Affairs – Scarborough Shoal incident diplomatic note
• US Indo-Pacific Command – Joint US-Philippines maritime patrol statement
• AMTI/CSIS – South China Sea Maritime Transparency Initiative: Scarborough Shoal incident report