Following World War I, many of the largest nations around the world sought a way to limit the likelihood that such a destructive war would ever happen again. Given the arms race in naval power that contributed to the start of World War I, one important way that this was accomplished was through efforts to limit the sizes of the navies of the largest powers. This was the aim of the Washington Naval Conference, a meeting held between the United States, the United Kingdom, Japan, France, and Italy. While the conference ended with a treaty, it also helped to plant the seeds for World War II in the Pacific.
Beginning in 1921, the United States sought to limit the power of the other two great naval powers of the day, the United Kingdom and Japan. In this spirit, diplomats decided that the best way to prevent an arms race between London and Washington was to cap the tonnage of warships at an equal amount. This was in recognition that the United States had to guard two oceans while the UK depended on its navy as a guarantee of security, with many strategic possessions in the Pacific.
Japan would be a more complicated issue. Tokyo was emerging as a serious power in the Pacific, and the United States sought to limit the expansion of the Japanese navy through the Pacific. Due to a deciphered communique from Tokyo to its negotiating team in Washington, the United States was able to find out the lowest ratio of tonnage that Japan would accept.
As a result, the Japanese accepted a ratio of 315,000 in battleship tonnage compared to 525,000 for the United States and the United Kingdom. France and Italy both had less pronounced need for naval power, and as such accepted smaller ratios. To help put this into perspective, the flagship of the British fleet, HMS Hood, was around 42,000 tons displaced.
A separate allowance of tonnage applied to the newly developed idea of aircraft carriers, with roughly the same ratios still in play. This helped to encourage the growth of the technology among the three great naval powers. Additionally, submarines faced less restrictive restrictions, with each country being permitted to build between 30 and 40.
By the end of the conference in 1922, two important legacies had already emerged. First and foremost, there were mixed feelings about the treaty in Japan. For many people, it was considered a point of considerable national pride that the country was considered to be at parity in the Pacific with the two largest naval powers. The Pacific was not a priority for either Washington or London, and as such Japan was able to match their combined forces. However, as time went on and militarism became more pronounced in Japan, there was resentment that the Western powers had limited Japan to any degree. This would be a popular rallying point for opposition not only against the treaty, but against the West in general.
Second, the limitation on battleship tonnage forced naval engineers to examine different technologies. The relatively loose restrictions on submarine construction encouraged large fleets of small, fast submarines, exactly the type that the United States would use in the Pacific War. Moreover, the ease with which large capital ships could be converted from battleships to aircraft carriers encouraged more research into how to make these ships a potent weapon of war. Most famously, the impact of this would be most acutely felt on a December Sunday in 1941, as well as throughout the rest of the Pacific Theatre during the war.