As North Korea continues to advance its nuclear capabilities along with long-range missiles and China’s potential invasion of Taiwan, along with U.S. military responses, dominates discussions on Far East Asian security, the recent establishment of the Trilateral Security Cooperation Framework has sparked questions about whether it includes detailed action plans for regional contingencies, such as simultaneous full-scale conflicts on the Korean Peninsula and in the Taiwan Strait. With the specifics of the Trilateral Security Cooperation Framework still confidential, and uncertainty surrounding whether it will evolve into an East Asian NATO equivalent (the Asia-Pacific Treaty Organization, a concept proposed by South Korean President Park Chung-hee in the 1960s but never realized), it is unclear how the three major democracies in the region — the United States, Japan, and South Korea — would collectively respond to such crises.
In the post-war period, security in the Far East was primarily ensured by two bilateral defense alliances: the U.S.-South Korean Mutual Defense Treaty and the U.S.-Japanese Security Treaty. While the former was designed to defend South Korea against a North Korean invasion, the latter aimed to protect Japan from external threats. However, the absence of a security treaty between Japan and South Korea made the connection between the two countries — the critical link within the tripartite relationship — particularly susceptible to domestic issues, especially historical disputes. As a result, the framework for security cooperation between Japan and South Korea had to be continuously shaped and reshaped by political leaders on both sides throughout the post-war era.
However, based on my research, as detailed in my doctoral dissertation “Japan’s Security Contribution to South Korea, 1950 to 2023,” the two countries demonstrated a significant level of security cooperation throughout the post-war period, even in the absence of a formal military alliance. For example, during the Korean War, Japan made substantial contributions to South Korean security despite not having formal diplomatic relations with South Korea at the time. With this context in mind, I aim to specifically explore Japan’s role in the event of an all-out war on the Korean Peninsula. What actions could Japan take in such a crisis, and what types of security support could it provide to South Korea?
To address these questions, I first examine the historical precedent of Japan’s actions during the Korean War, which remains the only instance of an all-out war on the Korean Peninsula in the post-war era. Secondly, I analyze Japan’s recent legislative development and integrate insights from interviews I conducted with over 20 Japanese security experts, including Akiyama Masahito, a former vice minister of defense, and Michishita Narushige, vice president of the National Graduate Institute for Policy Studies, as well as officials from the Self-Defense Forces. As a whole, this article presents an overview of the actions Japan could and is likely to take in the event of a full-scale war on the Korean Peninsula in 2024.
Japan in the Korean War
Shortly after North Korean troops crossed the 38th parallel on the morning of June 25, 1950, they employed their version of Blitzkrieg, forcing South Korean forces to retreat all the way to Pusan in the southeastern part of the peninsula. Although President Harry S. Truman announced on June 27, 1950 that U.S. forces would intervene under the auspices of a U.N. resolution, the United States faced challenges due to its geographical distance from Korea and the effects of significant arms reductions following World War II. As a result, the United States needed to identify a large logistics base and staging ground to deploy its forces in the region, with Japan emerging as the most suitable candidate.
Due to the limited territory available for U.N. forces to operate within the Pusan perimeter, Japan’s ports and transportation infrastructure became essential for transferring military hardware and non-military supplies from Japan to the Korean Peninsula, especially during the early phase of the war. After World War II, Japan’s ports, such as those in Yokohama, Kobe, and Kitakyushu, were revitalized and fully utilized by the U.S. military to transport troops, weapons, and supplies. The proximity of Japan’s western ports, such as Moji and Shimonoseki, to Korea made these hubs indispensable. In particular, maritime transportation was essential for conducting military operations, such as the Incheon landing in September 1950 and the Wonsan landing in October of the same year. Japanese crews, operating under U.S. command through the Naval Shipping Control Authority for Japanese Merchant Marine, transported thousands of U.S. and U.N. troops alongside vast quantities of military supplies. Japanese ships were also responsible for evacuating U.N. troops and Korean civilians during the Hungnam evacuation in December 1950, an operation that safely transported 17,500 vehicles, 105,000 soldiers, and 91,000 civilians.
Japan’s industrial capacity was another vital component of the U.N. war effort. Following the destruction of much of Korea’s infrastructure during the war, Japan became the primary site for repairing, refurbishing, and upgrading military equipment through Operation Rollup. This initiative, launched in 1947, aimed to regenerate surplus military equipment left in Japan and the Pacific after World War II. From June to October 1950, Japanese factories regenerated 15,000 general-purpose vehicles, 489,000 small arms, and 1,418 artillery pieces for use in the war. By the end of the war, Japan had repaired or rebuilt 65 percent of the military vehicles used in Korea. Japanese workers, supervised by U.S. military personnel, refurbished tanks, artillery, and other equipment that was crucial in the defense of Pusan and other early battles. M-4A3 Sherman tanks, initially introduced to Japan through Operation Rollup, were upgraded and sent to Korea, giving the U.N. forces much-needed firepower against North Korean forces. Aircraft were also repaired and upgraded in Japan. Japanese companies like Mitsubishi Heavy Industries and Kawasaki Aircraft refurbished U.S. bombers and fighter jets such as the B-29, F-51, and F-80. These aircraft, critical to U.S. air operations in Korea, were retrofitted with improved fuel systems and weapons. The regeneration program played a key role in maintaining U.S. air dominance throughout the war.
One of Japan’s most significant operational contributions during the Korean War was minesweeping. North Korea, supported by Soviet expertise, laid thousands of sea mines around key ports to block U.N. naval operations. Given the U.S. Navy’s limited minesweeping capabilities — having drastically reduced its fleet after World War II — Japan’s experienced minesweeping crews and vessels were indispensable. A total of 46 Japanese minesweepers and over 1,200 former Imperial Navy personnel were deployed to clear sea routes around Korean ports like Incheon, Wonsan, and Gunsan. These efforts enabled safe passage for U.N. ships, facilitating critical amphibious operations like the Incheon landing in September 1950. The clearance of sea mines also protected the transport of troops and supplies, ensuring the success of military campaigns and maintaining U.N. naval superiority.
In addition, Japan’s U.S. military bases were used extensively as launching pads for U.N. air, naval, and ground operations. Bases such as Itazuke Air Base and Kadena Air Base were critical for U.S. air dominance in the Korean War. U.S. fighter jets and bombers operated from Japanese airfields, launching missions against North Korean and Chinese forces. For instance, during the early phases of the war, B-29 bombers from Japan conducted bombing campaigns that targeted North Korean military installations.
In sum, Japan’s logistical and operational contributions during the Korean War were essential to the success of U.N. forces. Through transportation, equipment repair, minesweeping, and training, Japan played a multifaceted role that went beyond merely providing bases. Although Japan did not formally deploy troops under the U.N. flag, its security contributions were widely recognized by key figures like Gen. Douglas MacArthur, Gen. Matthew Ridgway, and Adm. Arleigh Burke, who witnessed the Korean War firsthand.
What About the Next Korean War?
Japan’s involvement in a second Korean War would likely be more extensive. Japan would play a critical support role in conjunction with the United States, focusing on missile defense, maritime security, and logistical assistance. Japan’s involvement in a conflict on the Korean Peninsula would likely exclude direct combat operations due to political sensitivities and legal restrictions. However, it would offer significant operational and logistical support through various activities. Beyond the 2015 U.S.-Japanese defense guidelines, the most reliable insight into Japan’s potential actions during a Korean contingency is found in Operation Plan 5055. Although this plan is confidential, some details have been disclosed in media reporting.
Operation Plan 5055, developed as an upgrade to Concept Plan 5055 after North Korea’s first nuclear test in 2006, is a comprehensive U.S.-Japanese defense strategy designed to address potential North Korean contingencies. It divides scenarios into two categories: one where Japan provides support without facing a direct attack, and another where Japan is directly attacked, particularly by ballistic missiles. The plan outlines detailed cooperation between U.S. and Japanese forces in areas such as situational awareness, logistics, command and control, and missile defense. It includes measures for search and rescue operations, protection of U.S. bases and ports, and rear-area support activities involving local authorities.
In the event that the latter scenario occurs, Japan’s ballistic missile defense system would play a crucial role in countering North Korean missile attacks. Given North Korea’s ability to launch missiles targeting not only South Korea but also U.S. military assets and territories in the region, such as Guam and Hawaii, Japan’s Aegis-equipped destroyers and land-based interceptors could play a critical role in intercepting these missiles. Japan’s ballistic missile defense capability would provide an additional layer of defense to U.S. forces and assets, ensuring their operational effectiveness during a Korean contingency. This would also help mitigate the potential for “nuclear decoupling” — the possibility that North Korea could drive a wedge between U.S. and South Korean/Japanese forces by threatening nuclear strikes, thereby weakening the coalition’s resolve.
Meanwhile, Japan would likely dispatch its minesweepers to Korean waters in such circumstances. North Korea is known to have deployed thousands of sea mines along its coastlines. These mines would pose a significant threat to U.S. and South Korean naval operations in the event of a conflict. Japan’s Maritime Self-Defense Force has a well-established minesweeping capability, as demonstrated during the International Mine Countermeasures Exercise 2013, and would likely be tasked with clearing sea routes in and around the Korean Peninsula. This operation would be crucial for ensuring the safe transport of U.S. reinforcements and supplies to the region, helping to facilitate landing operations in North Korea if necessary.
Japan’s role would extend to anti-air, anti-submarine, and anti-ship warfare. North Korea’s air force, though relatively outdated, could still pose a threat to reconnaissance and transport aircraft. Japanese fighters could help protect U.S. air assets from North Korean interceptors or aircraft attempting to infiltrate South Korean or Japanese airspace. Additionally, North Korea’s submarine fleet could attempt to disrupt sea lines of communication between Japan and South Korea, which are critical for the transport of U.S. military personnel and supplies. Japan’s Maritime Self-Defense Force, with its advanced anti-submarine capabilities, would be tasked with neutralizing North Korean submarines that pose a threat to these vital sea routes or to U.S. Navy vessels operating in the region.
Additionally, with Japan in the process of acquiring a counterstrike capability and planning to deploy Tomahawk cruise missiles from the United States by 2027, this new capability could be employed to strike North Korean missile launch sites or other key military targets in the event of a Korean contingency. This would align Japan with South Korea’s “kill chain,” which aims to neutralize North Korean missile threats preemptively. While Japan’s counterstrike capability is not yet fully operational, its development signals a shift towards a more proactive defense posture, one that could significantly enhance U.S. and South Korean efforts to disable North Korean military infrastructure during a conflict.
The protection of U.S. Maritime Prepositioning Ship Squadrons would be another critical role for Japan. These squadrons, which carry pre-positioned military supplies and equipment, are strategically stationed in locations such as Guam and Okinawa. In the early stages of a Korean contingency, these supplies could be rapidly deployed to South Korea, potentially even before a formal decision is made to send reinforcements from the U.S. mainland. Japan’s Maritime Self-Defense Force would likely be tasked with protecting these ships as they transit to South Korea, ensuring the quick deployment of essential military assets to the battlefield.
Japan’s logistical support, facilitated through its Self-Defense Forces, would be a key component of U.S. operations during a Korean contingency. Under Japan’s 2015 security legislation, the Self-Defense Force is authorized to provide rear-area support to U.S. forces, which could include the use of Japanese ports and airfields for resupply operations, medical support, and transport of personnel. Japan’s role in maintaining supply chains and providing logistical assistance, including the refueling and maintenance of U.S. military aircraft and ships, would help sustain U.S. military operations on the Korean Peninsula. Additionally, Japan could assist with the evacuation of non-combatants, including Japanese citizens and possibly U.S. civilians, from South Korea.
In addition to combat-related support, Japan could provide humanitarian assistance during a Korean contingency. This would include search and rescue operations for U.S. and allied personnel, as well as ship inspections to ensure compliance with international sanctions against North Korea.
Despite its enhanced capabilities, particularly in missile defense, minesweeping, and rear-area logistics, the chances of Japanese ground forces operating on South Korean soil remain slim. South Korea has traditionally opposed the presence of Japanese forces on its territory due to historical sensitivities, making it unlikely that the South Korean government would allow the Japan Self-Defense Forces to engage in direct combat operations on the peninsula. Several Korean military officials I interviewed noted that, in an extreme situation — such as the U.S.-South Korean combined forces being decimated by North Korean forces in the initial phase of the war and forced to retreat further south — the deployment of Japan’s Self-Defense Forces on Korean soil might become inevitable. However, due to political sensitivities, it remains highly unlikely that this scenario would occur.
Conclusion
If North Korea were to launch a full-scale invasion of South Korea at this point, it should anticipate Japan’s multifaceted involvement, ranging from missile defense and maritime security to logistical support for U.S. forces. Japan’s contribution would likely surpass its role during the Korean War, especially given its decision to acquire counterstrike capabilities and its right to exercise collective defense, enabled by the 2015 legislation. Japan is therefore expected to provide an unprecedented level of operational support to U.S. forces in a second Korean War. If Pyongyang’s leadership fully understands this, they may attempt to create divisions not only between the United States and South Korea, and the United States and Japan, but also between Japan and South Korea. Strengthening the trilateral alliance between the United States, Japan, and South Korea — and reinforcing the weakest link, Japanese-South Korean relations — is thus crucial.
Dr. Ju Hyung Kim is a defense analyst at a South Korean defense think tank and is currently in the process of transforming his doctoral dissertation, titled “Japan’s Security Contribution to South Korea, 1950 to 2023,” into a book.
Image: Navy Chief Petty Officer James Mullen via Wikimedia Commons
