The Washington Naval Treaty, like the
League of Nations and the Kellogg-Briand Pact, was one of the attempts in the wake of
World War I to secure peace through diplomacy. Since the naval arms race between Britain and Germany was seen as one of the triggers of the Great War, it seemed logical to the diplomats and politicians to ratchet down tensions by forcing the major naval powers into a construction straitjacket that would limit not only the numbers of capital ships (in terms of total tonnage) but impose a ten-year holiday on the construction of those ships.
U.S. Secretary of State Hughes originally proposed three points for the treaty:
- The aforementioned capital ship holiday
- The scrapping of existing or planned capital ships to bring signatories into a 5 (US):5 (UK):3 (Japan):1.75 (France):1.75 (Italy) tonnage ratio
- Ongoing limits of capital ship and secondary ship tonnage to the 5:5:3 ratio.
The limits were highly controversial in Britain and Japan; the former saw proponents of Royal Navy supremacy pitted against politicians trying to cut budgets to suit the postwar economic crisis as well as the public in Britain and the Dominions, which were unhappy with the high cost of the Royal Navy. Japan was less concerned about public opinion, having structured their constitutional monarchy among Prussian/German lines, but was wary of the expense involved in trying to outbuild the United States. The French objected to being reduced to the same tonnage as Italy, but were bought off by concessions on cruisers and submarines.
The limit on cruiser and destroyer tonnage also fell by the wayside; in its place came a limit on individual cruiser tonnage to 10,000 tons and armament to be no heavier than 8" guns. This suited the British, whose Hawkins class cruisers were then in construction; the Japanese, who were planning the Furutaka class, and the U.S., which was considering what it would need for operations in the Pacific. These limits gave rise to the term "Treaty cruisers", which referred to cruisers built in the interwar period conforming to the treaty limits. In a similar way, battleships were restricted to 35,000 tons each and no guns larger than 16"; aircraft carriers were limited to 27,000 tons and no more than ten 8" guns. There were exceptions made for carriers less than 10,000 tons, carriers then under construction (HMS Argus & Furious, and IJN Hosho), and the battleship IJN Mutsu, which had been paid for through public donatons, including gifts from Japanese schoolchildren. The UK and US were allowed to build equivalent ships. In addition, each signatory could convert two battleships or battlecruisers up to 33,000 tons into aircraft carriers which did not count against the limits.
Submarines were a contentious topic. Britain, which had suffered greatly from Germany's U-boats, demanded their abolition. None of the other powers liked that idea*, and after a British proposal to limit their total submarine tonnage to 90,000 tons with the other powers in proportion failed, the Treaty simply did not address the issue.
The treaty also addressed the issue of bases in the Pacific. Britain, Japan, and the U.S. agreed not to build new fortifications in the Pacific Ocean, which helped ease Japanese acceptance of the treaty since it effectively gave them naval superiority in the Western Pacific. Unofficially, the treaty also led to the end of the Anglo-Japanese Alliance, since the U.S. had made it clear that they wouldn't sign the Washington Naval Treaty unless Britain ended the alliance.
Finally, the treaty went into detail about which ships needed to be scrapped. The US scrapped or canceled thirty capital ships, the British 23, and Japan 17.
The Washington Naval Treaty was modified by the London Naval Treaties of 1930 and 1936, although the latter was redundant as Japan withdrew from the treaty in 1934 and Italy in 1936 as a result of League of Nations sanctions over its invasion of Ethiopia.
*Especially not the French, who had been talked into accepting their 1.75 capital ship ratio by the Treaty cruiser agreement and the promise of more submarines.