In 1930, at the Treaty for the Limitation and Reduction of Naval Armament held in the United Kingdom, Japan, still burdened with the repayment of war debts from the Russo-Japanese War, took a proactive stance towards realizing a reduction in military expenses through the conference. Similar to the earlier Washington Naval Treaty, which aimed to reduce capital ships such as battleships and aircraft carriers, the London event saw the participation of the five major naval powers: Japan, the United States, the United Kingdom, France, and Italy. However, France and Italy did not agree to the conditions and only partially participated.
During this conference, Japan's possession ratio of heavy cruisers was set at approximately half of that of the United States and the United Kingdom, totaling eight ships. On the other hand, light cruisers were allocated 50% compared to heavy cruisers for the US and the UK, destroyers were set at 70%, and submarines were guaranteed 10%, overall providing satisfactory terms for Japan. Notably, the construction capacity for light cruisers still had over 37.000 tons available, even after retaining two Tenryū-class and eight Kuma-class ships. Japan planned to utilize this capacity to build the Mogami-class cruiser, equipped with fifteen 15,5 cm guns. Although officially declared at 9.250 tons, the Mogami-class cruiser's actual displacement was similar to the Myōkō-class, with plans for future retrofitting to 23cm guns, akin to the Myōkō-class. Additionally, taking advantage of a loophole in the treaty that exempted warships with a standard displacement of 600 tons or less from restrictions, the Imperial Japanese Navy focused on developing small escort vessels with a combined capability of both 590-ton class subchasers and minesweepers, with a certain degree of seafaring ability. This aimed to strengthen escort forces to protect maritime traffic routes.
While the London Naval Treaty aimed to limit auxiliary vessels, the real challenge for Japan lay in the negotiations to come. Regarding aircraft carriers, which were subject to limitations similar to battleships in the Washington Naval Treaty, the initial proposal set the tonnage limits at 135.000 tons for the US and Britain, and 67.500 tons for Japan, with 60.000 tons for France and Italy.
Japan, deeply concerned about the effectiveness and threat of aviation in World War I, insisted on an aircraft carrier possession equal to that of the US and Britain. If Japan were to be at a disadvantage in carrier numbers against the US and Britain, effectively blocking the leap of enemy carriers would be nearly impossible, and Japan's homes, constructed mainly of paper and wood, would quickly be reduced to ashes if directly attacked. Furthermore, if Japan's maritime routes were attacked by carrier-based aircraft, the nation would be economically devastated in a short period. No matter how many battleships Japan possessed, a fleet engagement would be meaningless if the homeland was burning, and essential supplies were not reaching them. That was Japan's assertion.
However, during the conference at that time, Japan ultimately had to compromise, accepting a possession ratio of 50% compared to the US and Britain. Now, in this instance, Japan once again requested an increase in the number of aircraft carriers. However, the proposal was not for the traditional 10% possession but rather an increase from 50% to 60% compared to the US and Britain. The compromise offered was a reduction in the battleship possession ratio from 50% to 40% in comparison.
This seemed highly appealing to the US and Britain. Reducing Japan's battleship possession ratio from 50% to 40% meant that, in comparison to the fifteen ships of the US and Britain, Japan would have only six. Considering that it was unlikely that the new Fusō-class and Nagato-class battleships would be decommissioned, the likely candidates for reduction would be the surviving Kongō-class battlecruisers, namely, Haruna and Kirishima.
While these Kongō-class battlecruisers had weaknesses in defense, their high speed made them versatile and, from the perspective of the US and Britain, elusive and formidable adversaries. By accepting Japan's proposal, the US and Britain could force the decommissioning of these ships without any direct involvement. In the event of an actual war with Japan, the effort required to sink Haruna and Kirishima would be unknown. From this perspective, Japan's proposal was a significant advantage for the US and Britain.
Furthermore, aircraft carriers were ultimately only seen as auxiliary vessels supporting battleships. Additionally, even if Japan were to possess 60% of the aircraft carriers compared to the US and Britain, the technological disparity was too vast. Aircraft, being intricate machinery, prominently reflected the difference in scientific and industrial capabilities. Moreover, it was hard to imagine that the abilities of Asian pilots would surpass those of white pilots. Therefore, the qualitative advantage of the US and Britain should remain unshaken. Even if Japan were to possess the same number of aircraft carriers as the US and Britain, from the perspectives of industrial and scientific capabilities, as well as the human capabilities of the crew, there would be no significant issue. If anything, having 60% was even more advantageous.
In the end, albeit reluctantly, the US and Britain accepted Japan's proposal, secretly rejoicing. Therefore, they readily approved the change in classification of the two carriers, Hōshō and Ryūjō, from aircraft carriers to auxiliary ships. On the other hand, Japan's negotiating team, having secured almost a full response from the US and Britain in this disarmament conference, also silently celebrated this outcome as a victory.