Very nearly since the day Japan laid down JS Izumo (and her sister, Kaga) analysts have wondered whether the medium-sized aircraft carrier might eventually fly the F-35B Joint Strike Fighter (JSF). At 27,000 tons and 814 feet long, the Izumos are clearly large enough to operate the F-35B, the short take-off and vertical landing aircraft (STOVL) variant of JSF family. Moreover, Japan has recently expressed interest in acquiring a number of F-35Bs, officially for the purpose of operating from landing strips on small islands near the East China Sea.

Now, a source is reporting that the Izumo was in fact designed with the F-35B in mind. The Japanese Ministry of Defense has denied plans to retrofit Izumos to carry the fighter (they currently only carry helicopters), but Asahi Shimbun has reported that Izumos were, in fact, designed to carry the F-35B. Designating the ships “helicopter destroyers” is a way of avoiding controversy, domestic and international, over Japan’s possession of potentially offensive naval aviation capabilities.

As international subterfuge goes, this is really a mild case. But it’s hardly the first time that Japan has engaged in a bit of sleight-of-hand with respect to its carrier force. Prior to World War II, operating on limited funds and under the restrictions of the Washington Naval Treaty, the Imperial Japanese Navy (IJN) facilitated the construction of numerous ships designed for conversion into aircraft carriers. Many of these ships went on to play large roles in the campaigns of the Pacific War, although most were lost in the conflict.

Ryuho

The Washington Naval Treaty did not limit the construction of such supplementary vessels in the way that it limited carrier construction. But the IJN realized that it would need carriers more than it would need seaplane or submarine tenders. Accordingly, it designed numerous auxiliary ships with conversion in mind, the idea being to evade the terms of the treaty.

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The first surreptitious conversion was HIJMS Taigei. Constructed between 1933 and 1934, she served as a submarine tender during the Sino-Japanese War. Three days before Pearl Harbor, she was ordered to return to Yokosuka to begin conversion to an aircraft carrier. Ryuho (she acquired a new name during the process) re-entered service in November 1942. The results weren’t ideal; her hull strength and engine reliability were both low. She displaced 16,500 tons, but could only carry thirty-one aircraft. Ryuho was generally kept away from major combat operations, although she did participate in the Battle of Philippine Sea. She survived the war, although an air attack in March 1945 left her a burned out hulk.

Shoho and Zuiho

The IJN performed the same trick with Shoho and Zuiho, which began life as Tsurugizaki and Takasaki, another pair of submarine tenders. Only the former was completed in her original role, however. As aircraft carriers they were fast and useful, each displacing 11500 tons, and carrying thirty aircraft. Shoho was destroyed by American carrier aircraft at the Battle of Coral Sea; Zuiho, having served in most of the major engagements of the Pacific War, was sunk at the Battle of Leyte Gulf in 1944.

Chitose and Chiyoda

Around the same time as HIJMS Taigei, the IJN ordered a pair of seaplane carriers named Chitose and Chiyoda, with similar convertible characteristics. When the scarcity of carriers became clear during the Guadalcanal campaign, both ships underwent reconstruction. As carriers, Chitose and Chiyoda displaced 11,500 tons, and could each carry thirty aircraft. Both ships were destroyed by William Halsey’s 3rd Fleet at the Battle of Leyte Gulf, sacrificed as part of a diversionary operation.

Hiyo and Junyo

The biggest and most successful conversions didn’t even begin life as military vessels. The ships that would become Hiyo and Junyo began as large, fast luxury ocean liners. The IJN, with an eye towards supplementing its carrier fleet, subsidized the construction of the Izumo Maru and the Kashiwara Maru to the tune of 60 percent of total costs, and not coincidentally built a number of features into the hulls that enabled easy conversion into military vessels. The IJN formally acquired the ships during the construction process, and commissioned both in 1942. Large (24,000 tons) and capable of carrying a respectable air wing (fifty-five planes), both ships played a large role in the war from the Guadalcanal campaign onward. Hiyo was sunk by carrier aircraft at the Battle of Philippine Sea, while Junyo survived the war, albeit after taking three torpedoes from an American submarine in 1944.

Taiyo, Unyo, Chuyo, Kaiyo

Not even Hiyo and Junyo could sate the IJN’s appetite for convertible aircraft carriers. In deals similar to the one that brought the two fast carriers into the fleet, the IJN invested in four smaller cargo liners, to be built keel up with certain military modifications. Three of these became the Taiyo-class escort carriers, and a fourth slightly smaller, faster ship became HIJMS Kaiyo. All of the ships were relatively large (14,000 tons for Kaiyo, 20,000 tons for the others) but their slow speed limited them to escort duties. The three Taiyos entered service in 1941 and 1942; all were sunk by U.S. submarines in 1943 and 1944. Kaiyo, commissioned in 1943, barely survived the war after suffering a series of air attacks in port.

Conclusion

The intent of the IJN in building all of these vessels was subterfuge; at first it needed to avoid the restrictions imposed by the Washington Naval Treaty, and later it wanted to share costs with civilian ship operators while not tipping its hand about the size of the carrier fleet. And to be clear, Japan is not engaging in meaningful international legal subterfuge by incorporating offensive capabilities into Izumo. However, the government clearly wanted to avoid scrutiny from both domestic and international audiences by downplaying the carrier’s potential to operate offensive fighter aircraft. In part because of the history of Japanese naval aviation, the prospect of a strike carrier seems perilous in both Japan and China.

But even a passing glance at the size and speed of Izumo were sufficient to let anyone familiar with the existence of the F-35B know that the carriers might someday fly the fighters. Indeed, it would have been malpractice to build Izumo without some thought of the F-35B. Expense notwithstanding, the fighter offers a radical increase in offensive and defensive capability for a medium-sized carrier like JS Izumo. And while Japan’s Ministry of Defense may still publicly suggest that the F-35Bs are intended for small island landing strips, it seems likely that they will someday fly from JS Izumo and JS Kaga. The question then becomes whether Japan will take the next step and build a dedicated big carrier.

Robert Farley, a frequent contributor to TNI, is author of The Battleship Book. He serves as a Senior Lecturer at the Patterson School of Diplomacy and International Commerce at the University of Kentucky. His work includes military doctrine, national security, and maritime affairs. He blogs at Lawyers, Guns and Money and Information Dissemination and The Diplomat.

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