The Associated Press recently quoted Marston, director of the Southeast Asia Program at the Lowy Institute, as saying that the Taiwan Strait and the South China Sea are "closely related"; if Beijing blocks or seizes Taiwan by force, the impact may quickly spill over to the South China Sea. This judgment grasps the geographical reality, but it must add a limitation: the two seas are highly connected, which does not mean that any conflict will automatically trigger the other. A more precise statement is: same war zone, different fuses.
Taiwan is located at the juncture of the East China Sea, South China Sea and Philippine Sea. The south side and the Luzon Strait are important channels for China's navy and air force to enter and exit the Western Pacific. Beijing already has a submarine base in Hainan and artificial islands in the Spratlys, so controlling Taiwan is not the starting point for projecting military power. The real change is that Beijing may acquire ports, airports and sensing locations facing the deep-water Pacific, reducing the constraints on the central nodes of the island chain and making it more difficult for the United States, Japan and the Philippines to conduct anti-submarine warfare, early warning and reinforcements.
In the same way, if the United States declares that it will not cooperate in defense, it will not increase the number of PLA submarines overnight, but it will change the regional assessment of the United States' credit. The Philippines, Japan and Southeast Asian countries may accelerate armaments or turn to Beijing to avoid risks. This is credit spillover, not fire magic.
The base network has connected the two seas. Among the nine locations in the "Enhanced Defense Cooperation Agreement" between the United States and the Philippines, three in North Luzon face Taiwan and Balabac Island faces the South China Sea; the 2026 "Balabak" exercise also conducted exercises in both directions at the same time. However, the legal fuses are independent: the Taiwan Relations Act does not include an automatic troop dispatch clause; although the U.S.-Philippines Mutual Defense Treaty covers Philippine troops and official ships under armed attack in the South China Sea, the response must still follow constitutional procedures. Combat fusion cannot be misinterpreted as inevitable participation in war.
The economic chain is more direct. The Center for Strategic and International Studies estimates that in 2024, the Taiwan Strait and the Malacca Strait will each have more than 2. Four trillion U.S. dollars of goods pass through, accounting for approximately 21% of global seaborne trade; the two have overlapping cargo flows and cannot be added together to cause losses. Taiwan also controls more than 90% of the world's advanced process chip production capacity. Rhodium Group estimates that the blockade may interrupt more than $2 trillion in economic activity; this is an exposure amount, not an inevitable loss. What usually happens first is an increase in war risks, ship diversions and shrinking financing.
The United States' intervention is not only to "protect Taiwan", but also to prevent the first island chain from being dominated by a single power and to maintain alliances, supply chains and navigation order. However, clear interests do not mean unlimited resources; Washington’s recent transfer of Western Pacific warships to the Middle East shows that all theaters are still competing for the same batch of warships and aircraft.
Taiwan cannot treat the South China Sea as a distant place, nor can it treat its alliance as an automatic insurance policy. The top priority is to establish maritime surveillance, anti-submarine warning, evacuation, port and submarine cable repair coordination with Japan and the Philippines, and prepare energy, shipping and war risk backups; at the same time, clarify the crisis threshold to avoid gray friction from accidentally escalating into a full-scale conflict. If the Taiwan Strait is lost, the consequences will not stop at the coast of Taiwan; but geographical linkage does not mean the fate of war. The real deterrent is to allow the two seas to support each other, rather than letting one crisis drag the entire island chain out of control.
