During the London Naval Conference held in 1930 with the aim of restraining the naval vessels of the major naval powers, officials from the United States and the United Kingdom were perplexed by the proposals presented by the Japanese. Originally, the conference's main objective was to calm the escalating competition in the construction of auxiliary vessels such as cruisers and destroyers by establishing limits on their construction.
While there were heated debates about auxiliary vessels, the overall framework was set, and each country only needed to return home and obtain approval domestically. What had the American and British officials scratching their heads was Japan's sudden proposal for the number of capital ships it would possess. According to the Japanese representatives, they suggested reducing the battleship ownership ratio from 60% to 40% compared to the US and the UK, meaning they would decrease the number of battleships from nine to six. In return, they proposed increasing the number of aircraft carriers by that number.
"Deploying carrier-based attacks near Japan's mainland would quickly turn Japanese cities with many wooden houses into ashes. Battleships cannot catch fast carriers, and considering the national power of Japan, it is not realistic for them to defend all cities from aircraft operated by those carriers. Currently, the only means to counter carriers is with carriers, but to do so effectively, the appropriate quantity of carriers is necessary."
It was unclear how much of Japan's statements were truthful. However, officials from the US and the UK, intrigued by this proposal, asked the Japanese officials about the compensation for increasing the aircraft carrier ratio and which battleships would be decommissioned. In response, the Japanese officials mentioned the Fusō-class and Ise-class battleships. Both of these battleships were formidable, equipped with twelve 36 cm guns each, and in the event of a conflict with Japan, they would pose a significant threat following the Nagato-class. In the disarmament conference, where achieving results akin to sinking a ship with mere words was possible, it was worth considering.
Therefore, American and British officials discussed and created a condition: if Japan promised not to use the battleship quota for practice ships, they would agree to increase the aircraft carrier ownership ratio from sixty percent to the requested ten percent by the Japanese side as compensation. In other words, reduce the battleship quota by 105.000 tons, but in return, increase the aircraft carrier quota by 54.000 tons. The American and British officials thought Japan would not agree to such terms, but surprisingly, the Japanese side showed a willingness to compromise if certain conditions were met.
One of these conditions was the decommissioning of the aircraft carriers Akagi and Kaga. For a financially strapped Japan with inadequate shipbuilding facilities, these two carriers were considered burdensome due to their excessive size. Another condition was the conversion of the Hōshō into a repair ship. According to Japan, stating that the Hōshō, with a tonnage below 10.000 tons, was within the framework was, in a sense, a retrospective application of the law set during the Washington Naval Treaty.
In response to these Japanese demands, American and British officials readily agreed. It did not matter whether they kept or scrapped the Akagi and Kaga, whose performance was clearly inferior to that of the Lexington-class, and that it would be more painful for the US and UK if they made the scrapping of the Fusō and Ise class battleships a problem by complicating the discussion by making an issue of a small ship such as the Hōshō, which would result in the scrapping of the Fusō and Ise class ships.
Besides, even if Japan's aircraft carrier quota increases by 54.000 tons, it doesn't matter much. After all, 54.000 tons is at most the displacement of two large carriers or three medium carriers, and even if Japan were to possess carriers of the same displacement as the US and the UK, the performance of carrier-based aircraft, the crucial factor, would overwhelmingly favor them both. Moreover, compared to the UK, which will lose five battleships, or the US, which will lose three battleships in the London Naval Treaty, Japan should be able to maintain at least one battleship by converting it into a practice ship. In this conference, while it is indeed a notable achievement to restrict Japan's auxiliary vessels to less than 70% of the US and the UK, obtaining the commitment to decommission four battleships, including those in the practice ship quota, is significant.
"It's a major diplomatic victory."
Officials from the US and the UK were satisfied with the achievements they had outlined. However, this was a significant misunderstanding.