South China Sea: Simultaneous US-China Military

📡 THE SIGNAL — What Happened

Why it matters: In early 2026, the simultaneous deployment of large-scale military exercises by both the US and China in the South China Sea has created the most dangerous military tension since the Cold War. As the risk of accidental collision rapidly increases, the security order across the entire Indo-Pacific, including Japan, is being fundamentally questioned.

• Military — The U.S. Navy deployed two carrier strike groups to the South China Sea from January to March 2026 and conducted the joint exercise "Balikatan 2026" with the Philippine military. It involved the largest number of participating forces ever.
• Military — The Chinese People's Liberation Army conducted large-scale live-fire exercises in the South China Sea, primarily centered in the Southern Theater Command. Threatening exercises, including the launch training of Dongfeng-21D anti-ship ballistic missiles, were carried out.
• Diplomacy — At the ASEAN Foreign Ministers' Meeting, negotiations on the wording of a joint statement regarding tensions in the South China Sea faced difficulties, with Cambodia and Laos maintaining their pro-China stance.
• Security — Japan's Maritime Self-Defense Force dispatched the destroyer "Izumo" to the South China Sea and participated as an observer in the US-Philippines joint exercise, deepening its quasi-alliance involvement.
• Economy — Trade volume passing through the South China Sea reaches approximately $5.3 trillion annually, with about one-third of global maritime trade transiting this area.
• Legal — The 2016 South China Sea arbitration ruling rejected China's Nine-Dash Line, but China has consistently dismissed this ruling as "a piece of paper" and refused to comply.
• Technology — China has deployed HQ-9 long-range surface-to-air missile systems and YJ-62 anti-ship missiles on artificial islands in the Spratly Islands, accelerating the militarization of its effective control.
• Regional — The Philippines has repeatedly experienced physical clashes with the China Coast Guard during resupply missions to Second Thomas Shoal (Ayungin Shoal), with the situation escalating further since late 2025.
• Alliance — Regarding the scope of the US-Philippines Mutual Defense Treaty, the US explicitly stated it applies to armed attacks in the South China Sea. China condemned this as "adding fuel to the fire."
• Domestic — Within China, nationalist public opinion, centered on social media, asserting "the South China Sea is a core interest of China," is growing, putting pressure on the Xi Jinping administration.
• Economy — The seabed of the South China Sea is estimated to hold 11 billion barrels of oil and 190 trillion cubic feet of natural gas.
• Military — The US military is rapidly advancing the development of four new EDCA (Enhanced Defense Cooperation Agreement) sites within the Philippines.

The intensification of US-China confrontation in the South China Sea is not a sudden event but the result of decades of structural dynamics. To understand the root causes of this tension, it is necessary to trace historically the transformation of the post-Cold War international order and the geopolitical shifts brought about by China's rise.

The South China Sea territorial dispute dates back to the 1940s. The starting point was the Republic of China (later Taiwan) publishing the "Eleven-Dash Line" (later revised to the "Nine-Dash Line" by the People's Republic of China) in 1947, asserting sovereignty over almost the entire South China Sea. However, this issue only became a focal point for international security after China's rapid economic growth and military modernization began in earnest in the 2000s.

The 2012 Scarborough Shoal incident was a turning point. After a standoff with the Philippines, China established effective control over Scarborough Shoal, and the international community began to recognize the danger of the "salami-slicing" strategy—a method of altering the status quo by accumulating small faits accomplis. Subsequently, from 2013 to 2015, China undertook unprecedented large-scale artificial island construction in the Spratly Islands. Over 3,200 acres of artificial land were created on features such as Fiery Cross Reef, Subi Reef, and Mischief Reef, where runways, radar facilities, and anti-air/anti-ship missile systems were deployed. President Xi Jinping explicitly stated to President Obama in 2015 that China had "no intention to militarize the South China Sea," but this promise was completely broken.

The US response has also escalated incrementally. "Freedom of Navigation Operations (FONOPs)," initiated in 2015 under the Obama administration, increased in frequency during the first Trump administration and were further strengthened under the Biden administration. In a potential second Trump administration from 2024 onwards, a hardline stance against China has become a bipartisan consensus, and military presence in the South China Sea has reached its highest level ever, both in quality and quantity.

Behind this structural escalation are three major dynamics. First, the maturation of China's "Anti-Access/Area Denial (A2/AD)" strategy. With Dongfeng-21D and Dongfeng-26 anti-ship ballistic missiles, YJ-18 supersonic anti-ship cruise missiles, and the modernization of its submarine forces, China has significantly constrained the freedom of action for US forces within the first island chain. Second, the US shift to an "integrated deterrence" strategy. It is attempting to build a multi-layered deterrence network by leveraging its network of allies and partners, including AUKUS, the Japan-US-Australia-India (Quad), and the expansion of the US-Philippines EDCA. Third, the domestic politics of both countries demand a hardline stance externally. In China, the narrative of "the great rejuvenation of the Chinese nation" does not permit concessions in the South China Sea, while in the US, bipartisan hardline views on China narrow the president's room for dialogue.

The simultaneous military exercises in early 2026 are a consequence of these structural pressures. The US seeks to deter China's expansion in the "gray zone" through strengthening its alliance with the Philippines, while China aims to prevent neighboring countries from "bandwagoning" and to raise the cost of US intervention through military displays. The problem is that both strategies mutually reinforce the "security dilemma," confirming each other's worst assumptions. Both sides characterize their own actions as defensive, but they appear aggressive to the other. This cognitive gap is the greatest risk for escalating accidental clashes into intentional conflicts.

The delta: The simultaneous US-China military exercises in early 2026 signify a qualitative shift from the traditional asymmetric tension structure of "Freedom of Navigation Operations" versus "harassment by maritime militia" to a "symmetric escalation" where large-scale exercises by regular forces run concurrently in the same waters. This dramatically increases the probability of accidental collision and fundamentally tests the effectiveness of existing escalation management mechanisms (such as the US-China military hotline).