Armed Chinese Trainer Jets over Scarborough Shoal: Gray-Zone Competition in the South China Sea Expands to the Air Domain

Executive Summary

On September 2, China's Southern Theater Command deployed JL-10 trainer jets armed with PL-5 missiles to expel three Philippine light aircraft over Scarborough Shoal. This marks the latest phase in an escalating campaign of phased coercion, following China's establishment of de facto control over the feature in 2012 and its declaration of territorial sea baselines in 2024, now extending to control of the airspace above it. The use of trainer jets instead of fighters represents a low-cost, sustainable gray-zone tactic tailored to counter slow-moving turboprop aircraft, suggesting that similar incidents may become regular occurrences. As long as the asymmetric structure persists—with the Philippines holding legal legitimacy from the Hague ruling and China possessing physical force on the ground—collective ASEAN responses will remain limited, and U.S. intervention is likely to be confined to statements. South Korea should recognize this as a signal of a structural increase in SLOC risks in the South China Sea and establish a proactive response system, including constant monitoring, the activation of low-cost diplomatic channels, and risk notifications for relevant companies.

I. Situational Analysis of the Issue

The Incident of Armed Chinese Military Trainer Jets Expelling Philippine Aircraft over Scarborough Shoal: A Situational Analysis

1. Background and Developments

Scarborough Shoal is a coral reef formation located 240 km west of Luzon Island [3]. It is hundreds of kilometers from the Chinese mainland [3]. China has maintained de facto control over the feature since 2012. Although it was a traditional fishing ground for Filipino fishermen, access has been effectively blocked by the constant presence of the China Coast Guard [3].

In 2016, the Permanent Court of Arbitration in The Hague ruled that China's nine-dash line claim has no basis in international law [5]. The tribunal also ruled that no feature in the Spratly Islands can generate an exclusive economic zone or continental shelf [5]. China has not recognized this ruling [3][5]. Since the ruling, the situation in the South China Sea has solidified into a contest of strength on the ground, irrespective of legal arguments.

This dynamic has recently intensified. In June 2024, at Second Thomas Shoal, China Coast Guard personnel boarded a Philippine vessel while carrying bladed weapons [3]. In April 2025, the China Coast Guard landed on the uninhabited sandbar of Sandy Cay and raised the five-starred red flag [3]. On August 11, 2025, in the waters of Scarborough Shoal, a collision occurred as a China Coast Guard vessel and a PLA Navy destroyer were obstructing a Philippine Coast Guard operation, resulting in the deaths of two Filipino coast guard members [3]. China did not officially acknowledge the incident at the time but later honored its own personnel who died as having fallen in the line of duty [3]. The focus of the dispute has shifted from maritime rights to a contest for control over physical features, and now to claims of control over the airspace.

The Philippines, for its part, has pursued parallel legal responses. Following the amendment of its domestic laws concerning territorial sea baselines in 2024, it deposited its baselines with the UN in July 2026 [3]. This is an attempt to institutionally solidify the legal gains from the Hague ruling. As China's strengthening of its de facto control on the ground has proceeded in parallel with the Philippines' attempts at legal institutionalization, both sides have been engaged in a competition for legitimacy, each claiming a more solid basis for its position.

2. Current Situation

The September 2 incident is an example of this competition extending into the air domain. China's Southern Theater Command deployed at least three JL-10 trainer jets armed with PL-5 air-to-air missiles to drive away Philippine light aircraft—a BN-2A, a C-208, and a PA-31T—from the airspace over Scarborough Shoal [1]. Released footage shows Chinese electronic equipment identifying the three aircraft types and tracking one of them with a targeting reticle [1]. A warning message was also released: "Philippine aircraft, you have violated the airspace of China's Huangyan Dao" [1].

China's state-run Global Times described this as a "readiness patrol" by the Southern Theater Command [4]. It claimed this was a normal patrol activity conducted in the territorial sea and airspace of Huangyan Dao [4]. The paper used the exact phrase "Huangyan Dao is China's inherent territory" [4]. It also specified that the JL-10 appeared in the footage multiple times armed with live missiles [4]. The very fact that the Southern Theater Command did not conceal the incident but actively publicized it through its WeChat account shows that this action was not an accidental encounter but an intentional delivery of a message.

The SCMP assessed this as a "cost-effective" tactic [1]. The reason for deploying JL-10 trainers instead of fighter jets is that they can match the speed of the slow-moving turboprops and have lower operating costs per sortie [1]. A Type 052C destroyer, a Type 054A frigate, a Type 071 amphibious transport dock, and H-6K bombers were also reportedly mobilized. This suggests a two-tiered structure in which the low-cost coercion by trainer jets is backed by a show of force from conventional military assets.

According to ACLED data, Chinese troop movements from January to June 2026 increased by approximately 490% compared to the previous six months. These movements were concentrated in the areas of Scarborough Shoal, Second Thomas Shoal, and Thitu Island. This latest incident in the air should be seen as the most recent phase of this cumulative upward trend. On August 8, the U.S. State Department issued a statement to the effect that China was obstructing Filipino fishermen's access around Scarborough Shoal [3]. However, as the response has been limited to statements, it leaves room for China to frame this as "mere verbal support" [3].

3. Key Actors and Positions

China (Southern Theater Command)characterizes this incident as an act of defending territorial sovereignty [4]. Its claim to control the airspace, following its establishment of de facto control in 2012 and its declaration of territorial sea baselines in 2024, represents a phased expansion of jurisdictional claims from land to sea to air. The low-cost tactic of using trainer jets can be read as a calculated move to achieve the practical effect of expulsion while avoiding the risk of escalation associated with deploying regular fighter jets.

The Philippineshas a history of strongly opposing China's sovereignty narratives at the Department of National Defense level. When Chinese media recently claimed the Batanes region as Chinese territory, the Philippine Department of National Defense officially refuted it, labeling it "state-sponsored propaganda" and a violation of sovereignty [6]. The National Historical Commission also countered that Batanes has no historical connection to China or Taiwan [6]. Batanes abuts sea lanes near Taiwan, giving it great strategic significance [6]. These circumstances show Manila's concern that China's territorial claims are not limited to the Scarborough Shoal area but could extend to the northern Philippines. The Philippines, having had its light aircraft expelled, is responding with the legal card of depositing its territorial sea baselines with the UN, but its effective means of counter-response are limited due to the on-the-ground military disparity.

United StatesAs a defense treaty ally, holds the potential for intervention, but its response so far has been limited to State Department statements [3]. A U.S. defense media outlet assessed that the recently commissioned Chinese Type 054A frigate is alarming Japan, Taiwan, and the United States [9]. U.S. attention is focused less on the individual Scarborough incident and more on the overall pace of China's regional naval expansion.

Japanis not a direct party to the dispute but is working to strengthen the capabilities of regional partners by transferring decommissioned naval vessels to the Philippines and Indonesia. The Philippines is scheduled to receive five Abukuma-class frigates within the next two to three years [11]. This reflects Tokyo's strategy of bolstering the Philippines' maritime surveillance and response capabilities without explicitly demanding alignment against China [11].

ASEANhas been unable to establish a collective response mechanism due to conflicting interests among its member states [3]. Vietnam is separately demanding a halt to China's reclamation and construction activities in the Paracel Islands [13], indicating that ASEAN's internal interests on the South China Sea issue are fragmented across different features like the Spratlys, Paracels, and Scarborough Shoal. This reaffirms that unified pressure on China at the ASEAN level is structurally difficult.

4. Key Issues

First is the sustainability of low-cost coercion using trainer jets. If repeatable expulsion operations can be conducted at a lower cost per sortie than with fighter jets, this could become a new standard tactic for gray-zone pressure [1].

Second is the expansion of the dispute over territorial sea baselines into a claim of airspace control. The three-stage progression of China's actions—de facto control, declaration of baselines, and claims of airspace control—is expected to continue to clash with the Philippines' response of depositing its baselines with the UN [3].

Third is the quantitative increase in troop movements. The 490% increase according to ACLED data highlights a deterrence gap left by the U.S., whose response has been limited to statements [3]. The risk of accidental clashes was already confirmed by the August 2025 incident involving a collision between coast guard and naval vessels [3].

Fourth are the structural limitations of ASEAN's response. Due to the fragmentation of interests among member states, a collective resolution mechanism for the South China Sea issue is unlikely to function for the foreseeable future [3][13].

II. In-Depth Analysis of the Issue

The Incident of Armed Chinese Military Trainer Jets Expelling Philippine Aircraft over Scarborough Shoal: An In-Depth Analysis

1. Analysis of Root Causes

The immediate cause of this incident is simple. Three Philippine light aircraft flew over Scarborough Shoal, and China's Southern Theater Command expelled them, defining the act as a "violation of airspace" [1][4]. However, the root cause lies in the normative vacuum that has formed since the 2016 Hague arbitration ruling.

The Hague tribunal ruled that China's nine-dash line claim has no basis in international law [5]. It also determined that no feature in the Spratly Islands can generate an exclusive economic zone or a continental shelf [5]. However, this ruling lacked an enforcement mechanism. China rejected the ruling, calling it "null and void" and "not binding" [5]. Its defeat in international law did not lead to a retreat from its de facto control on the ground.

China has filled this gap by progressively expanding its de facto control. The progression from seizing Scarborough Shoal in 2012 to declaring territorial sea baselines in 2024, and now to claiming airspace control, is the latest phase of what CSIS has termed a "salami-slicing strategy" [5]. Each step, while individually not large enough to provoke all-out war, cumulatively expands the scope of de facto control from the territorial sea to the airspace. The Philippines' deposit of its territorial sea baselines with the UN in July 2026 is a legal response to this encroachment, but the legal deposit itself cannot reverse physical control on the ground.

To summarize the root cause in one sentence: it is an asymmetric structure where the party that lost its case under international law is nullifying the ruling with on-the-ground force, while the winning party has secured a legal basis but lacks the physical power to enforce it.

2. Structural Context

Security Structure: The Cost Asymmetry of the Gray Zone

China's use of JL-10 trainer jets instead of fighters in this incident has structural implications beyond a mere tactical choice. The JL-10 is originally a pilot training aircraft, but it was deployed on a combat mission armed with PL-5 air-to-air missiles [1][4]. The Philippine aircraft—the BN-2A, C-208, and PA-31T—are all slow-moving turboprops [1]. The trainer jets can match their speed while having lower sortie costs than fighters. From China's perspective, this is a choice that maintains the coercive effect while lowering operational costs, creating a "sustainable cost" structure that allows for the long-term repetition of gray-zone operations.

This asymmetry is also evident in the scale of the show of force, which mobilized a Type 052C destroyer, a Type 054A frigate, a Type 071 amphibious transport dock, and H-6K bombers. This is a method of constantly demonstrating an overwhelming power disparity while managing it at a level that does not escalate into an actual clash. The August 11, 2025, incident in which two Philippine Coast Guard members died in a collision with naval and coast guard vessels illustrates a failure point in this management [3]. It means that the routine use of coercive measures carries a latent risk of accidental escalation.

Political Structure: Combining Domestic Legitimacy and External Messaging

The Global Times' reporting of this operation as a "normal readiness patrol" and its use of the phrase "Huangyan Dao is China's inherent territory" demonstrate a structure that combines domestic public opinion management with an external deterrence message [4]. The Southern Theater Command's decision not to conceal the incident but to release video footage via WeChat serves the same purpose. It serves the dual function of reinforcing the narrative of territorial defense domestically while simultaneously sending a signal to the Philippines and the United States that "these waters and this airspace are under control."

The case of the Philippine Department of National Defense strongly refuting recent claims by Chinese media regarding Batanes as "state-sponsored propaganda" [6] shows that the narrative competition between the two countries is not confined to Scarborough Shoal but is expanding into a discursive offensive concerning Philippine territory as a whole. This is also linked to the structural isolation the Philippines faces within ASEAN. Due to conflicting interests among its member states, ASEAN lacks the capacity for a collective response to the South China Sea issue [3]. Vietnam's separate protests against reclamation activities in the Paracel Islands [13] are another example showing that responses remain sporadic and at the individual country level, rather than a joint effort at the ASEAN level.

Economic and Military Structure: The U.S. Intervention Dilemma

The United States has a mutual defense treaty with the Philippines, but its response to a series of gray-zone provocations, including this incident, has been limited to statements [3]. The U.S. State Department's August 8, 2026, statement criticizing China's obstruction of Filipino fishermen's access reaffirms this pattern [3]. Gray-zone provocations are designed to target precisely this point: they fall short of an "armed attack," the threshold for treaty activation, while still expanding de facto control. If the U.S. consistently responds only with statements, it becomes vulnerable to China's framing of it as "mere verbal support" [3]; yet, if it raises the level of its military response, it faces the dilemma of shouldering the risk of escalation itself.

3. Historical Precedents and Comparison with Similar Cases

This incident is not an isolated, sudden event but an extension of a cumulative pattern since 2012. It follows a path of step-by-step escalation: the start of de facto control over Scarborough Shoal in 2012; the creation of artificial islands and construction of military facilities from 2013 to 2015; the rejection of the Hague ruling in 2016; the boarding incident at Second Thomas Shoal in 2024; and the landing on Sandy Cay and the fatal collision involving coast guard and naval vessels in 2025 [3][5]. Each stage has expanded the domain of control beyond the previous one. The domain has shifted from restricting maritime fishing rights to occupying uninhabited features, then to physical clashes with manned vessels, and now to controlling overflights.

This pattern is similarly repeated in other disputed areas of the South China Sea. The reclamation of Antelope Reef and the installation of new facilities on Yagong Island in the Paracel Islands, which Vietnam protested [13], follow the same sequence as at Scarborough Shoal: gradual infrastructure development on small features is carried out first, which is then established as a fait accompli. The method of blocking Filipino fishermen's access by establishing a nature reserve, thereby circumventing accusations of violating international law [3], is another type of indirect tactic for establishing de facto control.

Viewed within the framework of rising military tensions between the U.S. and China, this incident can be seen as the South China Sea version of the gray-zone provocations that have been repeated in the Taiwan Strait and the East China Sea. The assessment that the deployment of Type 054A frigates is alarming Japan, Taiwan, and the U.S. simultaneously [9], and the instance of China conducting joint exercises with Egypt to demonstrate its military reach [7][12], both point to a common context: China is expanding its military presence on multiple fronts simultaneously, not just in the South China Sea. The announcement of joint exercises with Singapore [10], made around the same time, suggests that China is employing a strategy of managing regional countries individually through a combination of coercion and cooperation.

4. Key Variables Shaping Future Developments

First is the cumulative statistic on troop movements shown by ACLED data. The figure showing that Chinese troop movements from January to June 2026 increased by approximately 490% compared to the previous six months, concentrated in the areas of Scarborough Shoal, Sabina Shoal, and Thitu Island, supports the view that this incident was not a one-off provocation but an expression of an ongoing trend of strengthening military pressure. Whether this trend continues will be a primary indicator for gauging the situation over the next six months.

Second is whether the claim of airspace control becomes legally entrenched. If China repeatedly attempts to extend its de facto control from the territorial sea to the airspace and precedents are set by the international community's acquiescence, the legal significance of the territorial sea baselines deposited by the Philippines with the UN will be substantively eroded. The key will be how the Philippines officially raises this incident with the international community and the level of the international response.

Third is the level of the U.S. response. As long as the response remains at the level of statements, the "low-cost structure" of gray-zone coercion will be maintained. Conversely, if the U.S. expands joint patrols or asset deployments with the Philippines, it could change China's cost-benefit analysis, but this entails a trade-off of simultaneously increasing the possibility of accidental encounters.

Fourth is the manageability of the risk of accidental escalation. As shown by the fatal collision involving coast guard and naval vessels in August 2025, the possibility of unintended casualties or physical clashes arising from the routine use of gray-zone coercive measures is structurally ever-present. If such an incident recurs, the direction of the next phase will be determined by whether both countries exercise the restraint to moderate the intensity of coercion, or whether they escalate their responses under pressure from domestic public opinion.