Now entering its fifth year, the Ukraine War has become the largest armed conflict in Europe since World War II, developing into a high-intensity, prolonged war of attrition. Similarly to the two world wars fought on the European continent, the Ukraine War has taken on the characteristics of trench warfare, with Russia’s advance slowing to approximately 70 meters per day from February 2024 to January 2026, and the front line becoming largely fixed in eastern Ukraine. According to CSIS estimates, Russian forces suffered nearly 1.2 million battlefield casualties, which includes killed, wounded, and missing, between February 2022 and December 2025. In addition, Russia suffered between 275,000 and 325,000 fatalities over the same time period.[1] Military operations take different forms depending on the geographical environment. On the vast plains of the European continent, “land warfare” is centered on ground combat, and the place where soldiers hold out to the end becomes the ceasefire line and the territory. Even if the equipment deployed differs due to advances in science and technology, it is a geographical inevitability that the Russian and Ukrainian armies will repeat the same trench warfare as in the past.

Considering Taiwan’s geography, a Taiwan continency would become a “naval war” extending from mainland China to the entire Western Pacific Ocean, with Taiwan as the strategic target. In modern history, there have been three global-scale “naval wars”: World War I, World War II, and the Pacific War. Two countries were the strategic targets of offense and defense: the United Kingdom and Japan. Both of those countries are surrounded by the sea, have few domestic resources, and have geopolitical vulnerabilities, being deeply dependent on overseas trade for their survival and prosperity. In World War II and the Pacific War, fought by Japan and the United Kingdom at the same time, the battle over this vulnerability—that is, commerce raiding—significantly influenced the outcome of the wars on both the offensive and defensive sides.

Taiwan shares geopolitical vulnerabilities with Japan and the United Kingdom. If “naval wars” repeat the same battles of the past as a geographical inevitability, Taiwan can be expected to gain many insights from the lessons learned by Japan and the United Kingdom concerning commerce raiding.

The reasons why Japan ended up neglecting maritime traffic protection are intertwined with factors originating not only from Japan but also from the United States. Furthermore, the United Kingdom fought with the full and strong support of the British Commonwealth of Nations and the United States against a single enemy, Germany, whereas Japan had to fight on its own against the superior naval and air forces of the United Kingdom and the United States. In addition to the traditional sea areas around Japan north of Taiwan, the vast sea areas of the Dutch East Indies, the South China Sea, and the South Seas Mandate were added to the operational sea areas at the outbreak of the war. Therefore, while a simple comparison of the maritime traffic protection of Japan and the United Kingdom is not possible, it is meaningful to compare the two countries in the stage before the war broke out, as shown by the then Office of War History of the National Defense College concluding its Senshi-Sosho concerning maritime escort operations by stating that “Japan’s maritime escort operations already contained many elements that were destined to fail from the outset.”[2]

This paper compares the factors that led to differences in the approaches to maritime traffic protection between Japan and the United Kingdom, and discusses insights for Taiwan’s defense policy.

1. The Geopolitical Vulnerabilities of Japan and the United Kingdom: High Dependence on Overseas Resources

At the outbreak of World War II, the United Kingdom was dependent on foreign sources for all of its oil. In March 1936, when Germany invaded the Rhineland in violation of the Treaty of Versailles and began rearming, the United Kingdom decided to build a 90,000-ton protected oil storage facility in order to increase its aviation gasoline stocks, which stood at a mere 8,000 tons (enough for ten days in wartime).[3] When the Japanese military occupied U.K. oil fields in Burma in 1941, and the plentiful supplies from the vast Iranian concession were forced to take a long and hazardous transportation route to avoid the Mediterranean Sea, the United Kingdom became increasingly dependent on the United States and Venezuela for supplies.[4] At least 80% of the oil the United Kingdom imported was of Venezuelan origin[5] and the United Kingdom also depended on high-octane aviation fuel from the United States.[6] Regarding food, the United Kingdom had become accustomed to using the British Empire for food production, and was importing approximately 18 million tons of food per year in 1939. This included about 70% of its cheese and sugar, nearly 80% of fruits and about 70% of cereals and fats. The United Kingdom also imported more than half of its meat, and relied on imported feed to support its domestic meat production.[7] Therefore, it was obvious that if the United Kingdom were to go to war with Germany again, Germany would repeat the same commerce raiding tactics as in World War I in order to force the United Kingdom to surrender.

In the late 1930s, Japan was highly dependent on overseas sources for the strategic materials necessary for military operations. Materials more than 90% dependent on imports included oil, rubber, nickel, lead, cotton, and wool, and materials more than 50% dependent on imports included iron ore, zinc, tin, manganese, aluminum, and rare metals. The only materials that could be produced in sufficient quantities domestically were sulfur, coal, and steel.[8] The navy recognized early on that oil was a serious problem and it attempted to increase domestic oil reserves, but in 1936, the reserves were 3.5 million kiloliters, falling short of the estimated 3.6 million kiloliters needed in the first year of the war.[9] It was clear that if Japan went to war with great powers such as the United Kingdom and the United States, there would be a serious long-term shortage of oil and strategic materials, which were highly likely to be subject to embargoes. Japan was not self-sufficient in its own staple food, rice, and imported approximately 20% of it from Taiwan and Korea, so after the outbreak of war, there was a shortage of labor and materials, domestic agricultural production declined due to the weather conditions, and when imports from overseas were cut off, mainland Japan quickly fell into a food shortage.[10] J.E. Talbott carried out a comprehensive review of U.S. Navy documents, and pointed out that justifying unrestricted submarine warfare against Japan as retaliation for the attack on Pearl Harbor was merely a pretext, and that “unrestricted submarine warfare was the probable outcome of decisions made as early as 1919 and pursued throughout the interwar period.”[11] In other words, if a war broke out between Japan and the United States, there was a high likelihood that Japan would be subjected to commerce raiding by U.S. Navy submarines.

2. Preparation of the Maritime Escort Operations of the Navies of Japan and the United Kingdom

Both Japan and the United Kingdom recognized their geopolitical vulnerabilities and endeavored to mitigate them before the war, but there were large differences between the two countries regarding their preparations for maritime traffic protection (maritime escorts).

(1) The United Kingdom Prioritized Preparation of Maritime Escort Operations

The Royal Navy continued its research and development of anti-submarine warfare during the interwar period after World War I and completed the ASDIC anti-submarine sonar by mid-1938. In September 1939, it started requisitioning trawlers (83 by the end of 1941) and equipped them with ASDIC. Furthermore, upon the declaration of war, they launched plans to build many escort vessels suitable for convoy escorting, and the Admiralty controlled merchant ship operations.[12]

It is clear that the U.K. government’s emphasis on anti-submarine warfare stemmed from its experience of the nation facing starvation due to the German Navy’s unrestricted submarine warfare in World War I. The German Navy recognized that commerce raiding (guerre de course) using submarines (U-boats) was its greatest opportunity to subdue the United Kingdom, given the numerical inferiority of the German Navy. Its objective was to starve the United Kingdom before the United Kingdom could achieve victory over Germany through a naval blockade. The commerce raiding, which began in the English Channel, expanded its operational area (war zone) from the waters around Britain to the Mediterranean Sea and North Sea, and the operations escalated from capture in accordance with international law to unrestricted submarine warfare that violated international law. [13] As a result, by the end of the First World War, more than 3,000 British flagged merchant and fishing vessels had been sunk and nearly 15,000 merchant seamen had died.[14] Furthermore, 886,000 British military personnel died during that war, and the magnitude of these sacrifices deeply instilled a sense of war weariness in the consciousness of the British people.[15] Resurgent Germany’s rearmament and territorial expansion, which occurred less than 20 years after the end of World War I, undoubtedly greatly increased the government of the United Kingdom’s vigilance against commerce raiding by U-boats.

(2) Japan Delayed Preparation of Maritime Escort Operations

As soon as World War I broke out, the Imperial Japanese Navy established a Special European War History Investigation Department and dispatched a special investigation team to conduct a thorough investigation. Furthermore, at the request of the United Kingdom, it dispatched the Second Special Squadron to the Mediterranean Sea, successfully carrying out a total of 348 escorts and protecting a total of 788 ships from U-boat attacks, despite suffering killed, wounded, and sick personnel. [16] The lessons of World War I were well recognized even within the central command, and considering its own strategic environment, the Imperial Japanese Navy should have actively sought out its ally, the United Kingdom, for know-how in anti-submarine warfare, such as convoy escort systems, and sonar technology. However, attention was focused on the offensive capabilities of submarines instead, and the lessons of the maritime escort operations could not be fully utilized.[17] As a result, countermeasures in organization, education, operations, and equipment were all postponed, and serious efforts to strengthen maritime escort only began from two years after the start of the war, in mid-1943, when the Japanese merchant fleet was suffering significant and growing losses and ship transportation came under pressure. The Imperial Japanese Navy also lagged behind the Royal Navy in technological development. By the end of June 1940, sonar for use in anti-submarine warfare had been installed on a total of 42 destroyers and submarines, but the war began before it could truly be put to practical use as an underwater surveillance weapon.[18]