The Washington Naval Conference was a disarmament conference called by the United States and held in Washington, D.C., from November 12, 1921, to February 6, 1922. It was conducted outside the auspices of the League of Nations. It was attended by nine nations (the United States, Japan, China, France, Britain, Italy, Belgium, the Netherlands, and Portugal) regarding interests in the Pacific Ocean and East Asia. Germany was not invited to the conference, as it had already been disarmed under the terms of the Versailles Treaty. Soviet Russia was also not invited to the conference. It was the first arms control conference in history, and is still studied by political scientists as a model for a successful disarmament movement.
Held at Memorial Continental Hall, in Downtown Washington, it resulted in three major treaties: Four-Power Treaty, Five-Power Treaty (more commonly known as the Washington Naval Treaty), the Nine-Power Treaty, and a number of smaller agreements. These treaties preserved the peace during the 1920s but were not renewed in the increasingly hostile world of the Great Depression.
The world's popular mood was peace and disarmament throughout the 1920s. Women had just won the right to vote in many countries, and they helped convince politicians that money could be saved, votes won, and future wars avoided by stopping the arms race. Across the world, leaders of the women's suffrage movement formed international organizations such as the International Council of Women and the International Woman Suffrage Alliance. Historian Martin Pugh writes that they achieved the greatest influence in the 1920s, "when they helped to promote women's contribution to the anti-war movement throughout the Western world." In the United States, practically all the major Protestant denominations and highly-visible Protestant spokesmen were strong supporters of international peace efforts. They collaborated to work to educate their local congregations on the need for peace and disarmament.
At the end of World War I, the British still had the largest navy afloat, but its big ships were becoming obsolete, and the Americans and the Japanese were rapidly building expensive new warships. Britain and Japan were allies in a treaty that was due to expire in 1922. Although there were no immediate dangers, observers increasingly pointed to the American-Japanese rivalry for control of the Pacific Ocean as a long-term threat to world peace. By then, the British decided that it was better for them to cast their lot with Washington than Tokyo. To stop a needless, expensive, and possibly dangerous arms race, the major countries signed a series of naval disarmament agreements.
The American delegation, led by Secretary of State Charles Evans Hughes, included Elihu Root, Henry Cabot Lodge and Oscar Underwood, the last being the Democratic minority leader in the Senate. The conference's primary objective was to restrain Japanese naval expansion in the waters of the West Pacific, especially with regard to fortifications on strategically-valuable islands. Its secondary objectives were intended to obtain an ultimate limit to Japanese expansion and also an alleviation of concerns over possible antagonism with the British. They were to eliminate Anglo-American tension by abrogating the Anglo-Japanese alliance, to agree upon a favorable naval ratio vis-à-vis Japan, and to have the Japanese officially accept a continuation of the Open Door Policy in China.
The British, however, took a more cautious and tempered approach. Indeed, the British officials brought certain general desires to the conference: to achieve peace and stability in the West Pacific; avoid a naval arms race with the United States; thwart Japanese encroachment into areas under their influence; and preserve the security of Singapore, Hong Kong, and Dominion countries, but they did not enter the conference with a specific laundry list of demands. Rather, they brought with them a vague vision of what the West Pacific should look like after an agreement.
Japanese officials were more focused on specifics than the British, and they approached the conference with two primary goals: to sign a naval treaty with Britain and the United States and to obtain official recognition of Japan's special interests in Manchuria and Mongolia. Japanese officials also brought other issues to the conference: a strong demand to remain in control of Yap, Siberia, and Tsingtao as well as more general concerns about the growing presence of American fleets in the Pacific.
The American hand was strengthened by the interception and decryption of secret instructions from the Japanese government to its delegation. The message revealed the lowest naval ratio that would be acceptable to Tokyo; US negotiators used that knowledge to push the Japanese. This success, one of the first in the US government's budding eavesdropping and cryptology efforts, led eventually to the growth of such agencies.
The head of the Japanese delegation to the Washington Naval Conference was Prince Iyesato Tokugawa, who during the first four decades of the twentieth century led a political movement in Japan that promoted democracy and international goodwill with the U.S., Europe and Asia. His influence was significant in the negotiations and ratification of the Washington Naval Treaty.
US President Warren Harding called the Washington Conference a deal that all countries thought best for themselves. To resolve technical disputes about the quality of warships, the conferees adopted a standard based on the tonnage displacement, a simple measure of the size of a ship. A ten-year agreement fixed the ratio of battleships at 5:5:3: 525,000 tons for the US, 525,000 tons for Britain, and 315,000 tons for Japan. Smaller limits with a ratio of 1.67 applied to France and Italy. Battleships, the dominant weapons systems of the era, could be no larger than 35,000 tons. The major powers allowed themselves 135,000:135,000:81,000 tons for the newly-developed aircraft carriers.
While the admirals were unhappy, peace activists strongly supported the results and successfully worked for ratification. In the United States they included the World Peace Foundation; the American Association for International Conciliation; the Carnegie Endowment for International Peace; the Women's Peace Society; the Women's World Disarmament Committee; the Women's International League for Peace and Freedom, and the Federal Council of Churches of Christ in America. 
The Washington Conference exactly captured the worldwide popular demand for peace and disarmament. Without an agreement, the US, Britain and Japan would have engaged in an expensive buildup, with each fearing the other two getting too powerful. However, even with the restrictions, the agreement solidified Japan's position as a great power and was treated as a colonial power with equal diplomatic interests, a first for a non-Western nation.
The naval treaty was concluded on February 6, 1922. Ratifications of the treaty were exchanged in Washington on August 17, 1923, and it was registered in League of Nations Treaty Series on April 16, 1924.
Japan agreed to revert Shandong to Chinese control by an agreement concluded on February 4, 1922. Ratifications of the agreement were exchanged in Beijing on June 2, 1922, and it was registered in League of Nations Treaty Series on July 7, 1922.
The Washington Naval Treaty led to an effective end to building new battleship fleets, and the few ships that were built were limited in size and armament. Many existing capital ships were scrapped. Some ships under construction were turned into aircraft carriers instead.
Even with the treaty, the major navies remained suspicious of one another and briefly (1927–1930) engaged in a race to build heavy cruisers, which had been limited in size (10,000 tons) but not numbers. That oversight was resolved on value of cruisers by the London Naval Treaty of 1930, which specified a 10:10:7 ratio for cruisers and destroyers. For the first time, submarines were also limited, with Japan given parity with the US and Britain, at 53,000 tons each. (Submarines typically displaced 1,000-2,000 tons each.) The US Navy maintained an active building program that replaced obsolescent warships with technically more sophisticated new models in part because its construction yards were important sources of political patronage and so were well protected by Congress. During the New Deal, relief funds were used to build more warships. "The naval program was wholly mine," President Franklin Roosevelt boasted.
The pacts and the treaties that resulted from the Washington Naval Treaty remained in effect for fourteen years. Japan ended participation in 1936.