Countdown to Collapse: Why Japan and South Korea Must Secure Their Sea Lanes in a Taiwan Contingency
Countdown to Collapse: Why Japan and South Korea Must Secure Their Sea Lanes in a Taiwan Contingency
The MOC
By Dr. Ju Hyung Kim
September 3, 2025
When the first Chinese warships move to blockade Taiwan, the world’s eyes will be on the fate of the island’s democracy—and on whether U.S. forces intervene. But a quieter, equally dangerous countdown will begin in Tokyo and Seoul. That clock will not be measured in missiles or battleships, but in barrels of oil and cubic meters of LNG. And if Japan and South Korea cannot secure their sea lines of communication (SLOC) within 30 to 45 days, their economies could face noticeable difficulties—before the first shot is fired in their direction.
The stakes are clear. Both nations are maritime trading powers with extreme dependence on imported energy. Nearly 90 percent of Japan’s and South Korea’s energy imports transit through vulnerable chokepoints such as the Strait of Malacca, South China Sea, East China Sea, and waters near Taiwan—many of which would fall within or near a conflict zone if China imposes a naval blockade or missile interdiction campaign around Taiwan.
How Long Can They Hold?
Japan maintains one of the world’s largest strategic petroleum reserves, holding the equivalent of 232 days of net oil imports. But its LNG reserves—used for power generation—last just 19 days. Industrial sectors like semiconductors and automotive manufacturing depend heavily on stable energy flows and just-in-time imports of critical materials, many from Taiwan.
Meanwhile, South Korea’s energy infrastructure is more vulnerable. Its strategic petroleum reserve is around 207 days, and its LNG reserves are equal to 30 days, which means that Seoul’s energy lifeline would be severed immediately, since unlike Japan, South Korea has less ability to substitute oil for LNG in electricity generation. Using the Sunda Strait and Lombok Strait would cover a longer distance, entail greater expenses, and be more vulnerable to outside disruption.
Such stark reality comes with a hard deadline. If Japan and South Korea are to avoid strategic paralysis, they should swiftly restore SLOC security once a Taiwan contingency occurs. Such measures include securing the LNG tankers that could divert through alternative maritime routes, deterring the PLA Navy’s disruption, and guaranteeing the safe maritime passage of commercial vessels in a more contested maritime area.
What Can Their Navies Do?
Despite historical and political differences, the Japanese Maritime Self-Defense Force (JMSDF) and the Republic of Korea Navy (ROKN) are among the most modern and capable in the Indo-Pacific. Both navies already conduct anti-piracy operations, joint exercises with the United States, and have deployed far from their shores.
In a Taiwan contingency, Japan and South Korea could align—most likely under a U.S.-led Combined Maritime Task Force—to conduct five core naval missions essential for maintaining regional stability and securing critical sea lines of communication.
First, the navies of both countries could embark on a maritime patrol and escort operation for commercial vessels and LNG tankers that would take longer alternative routes. Since the existing naval platforms possess both capacity and experience, such operations are well within reach of execution.
Second, both countries would cooperate on ISR operations and maritime domain awareness. Real-time intelligence is very important in major areas like the Philippine Sea and Bashi Strait, and would likely be facilitated by data fusion centers run by the U.S. and secure US-Japan-ROK coordination mechanisms.
Third, bearing in mind that Chinese submarines pose a threat to SLOCs, cooperation in the ASW domain would be crucial. Japan already possesses the most advanced anti-submarine assets in the region, while South Korea has been reinforcing its state of readiness through the introduction of P-8 maritime patrol aircraft and the deployment of KDX-III destroyers.
Fourth, the two countries could conduct long-term operations by sharing maritime logistics supply and repair hubs. For example, using Japan’s Sasebo and South Korea’s Jinhae naval base would extend the operational sustainability, yet such measures would require political consensus and U.S. logistical support.
Last but not least, the two navies could conduct joint minesweeping operations in order to disable sea mines laid by China. Japan and South Korea both field modern minesweepers and have experience clearing contested or hazardous maritime zones.
How Much Can They Deploy?
Operational reality limits what either country can commit. Factoring in maintenance cycles and home defense requirements, Japan could realistically deploy 12–16 destroyers, 2–3 submarines, and over 20 ASW aircraft to the Taiwan theater. South Korea could contribute 6–8 destroyers, 1–2 submarines, and 6–10 ASW aircraft. Although both navies could operate amphibious ships and support vessels, neither of the two sides possesses a comprehensive blue-water replenishment capability that would be essential for long-term operations; U.S. logistical support would be a sine qua non.
Yet such implementation would entail a cost burden. Both Japan and South Korea should maintain their respective coastal defense, given the threat in the vicinity (especially from North Korea). With around 70 submarines (although most of them are outdated, they still pose a threat), North Korea could conduct a provocation, an incursion, or a covert maritime operation, exploiting the Taiwan contingency. Thus, South Korea could not take the risk of completely redeploying high-performing submarines from both the western and eastern coasts of South Korea.
In order to manage the right balance, both Japan and South Korea should maintain a defense posture; both should allocate assets for SLOC protection, yet they should spare sufficient assets—second-line forces, submarines, and airborne anti-submarine platforms—for homeland defense. South Korea’s FFX class frigates and 18 submarines are indispensable for patrolling the Yellow and East Seas, while Japan’s P-1 and P-3C aircraft and around 20 submarines would continue to maintain surveillance around major maritime points near Okinawa and the Tsushima Strait.
U.S. Leadership Still the Keystone
Even the most sophisticated Japan–ROK naval cooperation will falter without a U.S.-led framework. Bilateral interoperability remains limited, and while Japan has loosened some constitutional restrictions on overseas operations through recent security legislation, political caution remains. That’s why any contingency planning must assume U.S. central command, pre-authorized rules of engagement, and coalition naval operations modeled after Operation Sharp Guard.
Yet Washington’s ability to lead such efforts may itself be stretched. A Taiwan contingency could demand the full commitment of U.S. naval assets, leaving fewer resources to coordinate regional convoy protection. If North Korea takes advantage of the crisis with provocations or missile threats, the U.S. may have to divide its attention—placing even greater responsibility on Japan and South Korea to act.
No Time to Waste
The real danger for Japan and South Korea is not merely that war breaks out—it is that they fail to act swiftly enough in its opening weeks. Energy security is national security. A Taiwan conflict could paralyze Tokyo and Seoul not through kinetic strikes, but through slow economic suffocation.
By planning now for joint SLOC security operations—including legal frameworks, logistical hubs, force apportionment, and command integration—Japan and South Korea can ensure that in a crisis, their navies sail not in isolation, but as a coalition capable of keeping their economies—and their societies—alive.
Dr. Ju Hyung Kim, President of the Security Management Institute—a defense think tank affiliated with the South Korean National Assembly—led a study commissioned by the South Korean Navy titled ‘Regional Naval Modernization Trends and Future Directions for the ROK Navy’s Core Capabilities.’
The views expressed in this piece are the sole opinions of the author and do not necessarily reflect those of the Center for Maritime Strategy or other institutions listed.
By Dr. Ju Hyung Kim
When the first Chinese warships move to blockade Taiwan, the world’s eyes will be on the fate of the island’s democracy—and on whether U.S. forces intervene. But a quieter, equally dangerous countdown will begin in Tokyo and Seoul. That clock will not be measured in missiles or battleships, but in barrels of oil and cubic meters of LNG. And if Japan and South Korea cannot secure their sea lines of communication (SLOC) within 30 to 45 days, their economies could face noticeable difficulties—before the first shot is fired in their direction.
The stakes are clear. Both nations are maritime trading powers with extreme dependence on imported energy. Nearly 90 percent of Japan’s and South Korea’s energy imports transit through vulnerable chokepoints such as the Strait of Malacca, South China Sea, East China Sea, and waters near Taiwan—many of which would fall within or near a conflict zone if China imposes a naval blockade or missile interdiction campaign around Taiwan.
How Long Can They Hold?
Japan maintains one of the world’s largest strategic petroleum reserves, holding the equivalent of 232 days of net oil imports. But its LNG reserves—used for power generation—last just 19 days. Industrial sectors like semiconductors and automotive manufacturing depend heavily on stable energy flows and just-in-time imports of critical materials, many from Taiwan.
Meanwhile, South Korea’s energy infrastructure is more vulnerable. Its strategic petroleum reserve is around 207 days, and its LNG reserves are equal to 30 days, which means that Seoul’s energy lifeline would be severed immediately, since unlike Japan, South Korea has less ability to substitute oil for LNG in electricity generation. Using the Sunda Strait and Lombok Strait would cover a longer distance, entail greater expenses, and be more vulnerable to outside disruption.
Such stark reality comes with a hard deadline. If Japan and South Korea are to avoid strategic paralysis, they should swiftly restore SLOC security once a Taiwan contingency occurs. Such measures include securing the LNG tankers that could divert through alternative maritime routes, deterring the PLA Navy’s disruption, and guaranteeing the safe maritime passage of commercial vessels in a more contested maritime area.
What Can Their Navies Do?
Despite historical and political differences, the Japanese Maritime Self-Defense Force (JMSDF) and the Republic of Korea Navy (ROKN) are among the most modern and capable in the Indo-Pacific. Both navies already conduct anti-piracy operations, joint exercises with the United States, and have deployed far from their shores.
In a Taiwan contingency, Japan and South Korea could align—most likely under a U.S.-led Combined Maritime Task Force—to conduct five core naval missions essential for maintaining regional stability and securing critical sea lines of communication.
First, the navies of both countries could embark on a maritime patrol and escort operation for commercial vessels and LNG tankers that would take longer alternative routes. Since the existing naval platforms possess both capacity and experience, such operations are well within reach of execution.
Second, both countries would cooperate on ISR operations and maritime domain awareness. Real-time intelligence is very important in major areas like the Philippine Sea and Bashi Strait, and would likely be facilitated by data fusion centers run by the U.S. and secure US-Japan-ROK coordination mechanisms.
Third, bearing in mind that Chinese submarines pose a threat to SLOCs, cooperation in the ASW domain would be crucial. Japan already possesses the most advanced anti-submarine assets in the region, while South Korea has been reinforcing its state of readiness through the introduction of P-8 maritime patrol aircraft and the deployment of KDX-III destroyers.
Fourth, the two countries could conduct long-term operations by sharing maritime logistics supply and repair hubs. For example, using Japan’s Sasebo and South Korea’s Jinhae naval base would extend the operational sustainability, yet such measures would require political consensus and U.S. logistical support.
Last but not least, the two navies could conduct joint minesweeping operations in order to disable sea mines laid by China. Japan and South Korea both field modern minesweepers and have experience clearing contested or hazardous maritime zones.
How Much Can They Deploy?
Operational reality limits what either country can commit. Factoring in maintenance cycles and home defense requirements, Japan could realistically deploy 12–16 destroyers, 2–3 submarines, and over 20 ASW aircraft to the Taiwan theater. South Korea could contribute 6–8 destroyers, 1–2 submarines, and 6–10 ASW aircraft. Although both navies could operate amphibious ships and support vessels, neither of the two sides possesses a comprehensive blue-water replenishment capability that would be essential for long-term operations; U.S. logistical support would be a sine qua non.
Yet such implementation would entail a cost burden. Both Japan and South Korea should maintain their respective coastal defense, given the threat in the vicinity (especially from North Korea). With around 70 submarines (although most of them are outdated, they still pose a threat), North Korea could conduct a provocation, an incursion, or a covert maritime operation, exploiting the Taiwan contingency. Thus, South Korea could not take the risk of completely redeploying high-performing submarines from both the western and eastern coasts of South Korea.
In order to manage the right balance, both Japan and South Korea should maintain a defense posture; both should allocate assets for SLOC protection, yet they should spare sufficient assets—second-line forces, submarines, and airborne anti-submarine platforms—for homeland defense. South Korea’s FFX class frigates and 18 submarines are indispensable for patrolling the Yellow and East Seas, while Japan’s P-1 and P-3C aircraft and around 20 submarines would continue to maintain surveillance around major maritime points near Okinawa and the Tsushima Strait.
U.S. Leadership Still the Keystone
Even the most sophisticated Japan–ROK naval cooperation will falter without a U.S.-led framework. Bilateral interoperability remains limited, and while Japan has loosened some constitutional restrictions on overseas operations through recent security legislation, political caution remains. That’s why any contingency planning must assume U.S. central command, pre-authorized rules of engagement, and coalition naval operations modeled after Operation Sharp Guard.
Yet Washington’s ability to lead such efforts may itself be stretched. A Taiwan contingency could demand the full commitment of U.S. naval assets, leaving fewer resources to coordinate regional convoy protection. If North Korea takes advantage of the crisis with provocations or missile threats, the U.S. may have to divide its attention—placing even greater responsibility on Japan and South Korea to act.
No Time to Waste
The real danger for Japan and South Korea is not merely that war breaks out—it is that they fail to act swiftly enough in its opening weeks. Energy security is national security. A Taiwan conflict could paralyze Tokyo and Seoul not through kinetic strikes, but through slow economic suffocation.
By planning now for joint SLOC security operations—including legal frameworks, logistical hubs, force apportionment, and command integration—Japan and South Korea can ensure that in a crisis, their navies sail not in isolation, but as a coalition capable of keeping their economies—and their societies—alive.
Dr. Ju Hyung Kim, President of the Security Management Institute—a defense think tank affiliated with the South Korean National Assembly—led a study commissioned by the South Korean Navy titled ‘Regional Naval Modernization Trends and Future Directions for the ROK Navy’s Core Capabilities.’
The views expressed in this piece are the sole opinions of the author and do not necessarily reflect those of the Center for Maritime Strategy or other institutions listed.