The Washington Naval Treaty, also known as the Five-Power Treaty, was signed during 1922 among the major Allies of World War I, which agreed to prevent an arms race by limiting naval construction. It was negotiated at the Washington Naval Conference in Washington, D.C., from November 1921 to February 1922 and signed by the governments of the British Empire (including the United Kingdom, Canada, Australia, New Zealand, South Africa and India), United States, France, Italy, and Japan. It limited the construction of battleships, battlecruisers and aircraft carriers by the signatories. The numbers of other categories of warships, including cruisers, destroyers, and submarines, were not limited by the treaty, but those ships were limited to 10,000 tons displacement each.
The treaty was finalized on February 6, 1922. Ratifications of it were exchanged in Washington on August 17, 1923, and it was registered in the League of Nations Treaty Series on April 16, 1924.
Later conferences on naval arms limitation sought additional limitations of warship building. The terms of the Washington Naval Treaty were modified by the London Naval Treaty of 1930 and the Second London Naval Treaty of 1936. By the mid-1930s, Japan and Italy had renounced the treaties, while Germany renounced the Treaty of Versailles which had limited its navy. Naval arms limitation became increasingly difficult for the other signatories.
Immediately after World War I, Britain still had the world's largest and most powerful navy, followed by the United States and, more distantly, by Japan, France and Italy. The British Royal Navy interned the defeated German High Seas Fleet in November 1918. The Allies had differing opinions concerning the final disposition of the Imperial German Navy, with the French and Italians wanting the German fleet divided between the victorious powers and the Americans and British wanting the ships destroyed. The negotiations became mostly moot after the German crews scuttled most of their ships on June 21, 1919.
News of the scuttling angered the French and the Italians, with the French particularly unimpressed with British explanations that the fleet guarding the Germans had then been away on exercises. Nevertheless, the British joined their allies in condemning the German actions, and no credible evidence emerged to suggest that the British had collaborated with the Germans with respect to the scuttling. The Treaty of Versailles, signed a week later on June 28, 1919, imposed strict limits on the sizes and numbers of warships which the newly-installed German government had the right to build and maintain.
The Americans, British, French, Italians, and Japanese had been allies during World War I, but with the German threat seemingly finished, a naval arms race between the allies seemed likely. US President Woodrow Wilson's administration had already announced successive plans for the expansion of the US Navy from 1916 to 1919 that would have resulted in a massive fleet of 50 modern battleships.
In response, the Japanese Diet in 1920 finally authorised construction of warships to enable the Imperial Japanese Navy to attain its goal of an "eight-eight" fleet programme, with eight modern battleships and eight battlecruisers. The Japanese started work on four battleships and four battlecruisers, all of which were much larger and more powerful than those of the classes that they were replacing.
The 1921 British Naval Estimates planned four battleships and four battlecruisers, with another four battleships to follow the subsequent year.
The new arms race was unwelcome to the American public. The US Congress disapproved of Wilson's 1919 naval expansion plan, and the 1920 presidential election campaign resulted in politicians in Washington resuming the non-interventionism of the prewar era, with little enthusiasm for continued naval expansion. Britain also could ill afford the exorbitant cost of capital ships.
In late 1921, the US became aware that Britain was planning a conference to discuss the strategic situation in the Pacific and Far East regions. To forestall the British plan and to satisfy domestic demands for a global disarmament conference, Warren Harding's administration called the Washington Naval Conference in November 1921.
The Conference agreed to the Five-Power Naval Treaty as well as a Four-Power Treaty on Japan and a Nine-Power Treaty on China.
At the first plenary session held November 21, 1921, US Secretary of State Charles Evans Hughes presented his country's proposals. Hughes provided a dramatic beginning for the conference by stating with resolve: "The way to disarm is to disarm". The ambitious slogan received enthusiastic public endorsement and likely abbreviated the conference while helping ensure his proposals were largely adopted. He subsequently proposed the following:
A ten-year pause or "holiday" of the construction of capital ships (battleships and battlecruisers), including the immediate suspension of all building of capital ships.
The scrapping of existing or planned capital ships to give a 5:5:3:1.67:1.67 ratio of tonnage with respect to Britain, the United States, Japan, France and Italy respectively.
Ongoing limits of both capital ship tonnage and the tonnage of secondary vessels with the 5:5:3 ratio.
The proposals for capital ships were largely accepted by the British delegation. However, they were controversial with the British public. Britain could no longer have adequate fleets in the North Sea, the Mediterranean and the Far East simultaneously, which provoked outrage from parts of the Royal Navy.
Nevertheless, there was huge demand for the British to agree to the limits and reductions: the risk of war with the Americans was increasingly regarded as merely theoretical as there were very few policy differences between the two Anglophone powers; continued naval spending was unpopular in Britain throughout the empire; and Britain was implementing major budget reductions due to the post–World War I recession.
The Japanese delegation was divided. Japanese naval doctrine required the maintenance of a fleet 70% the size of that of the United States, which was felt to be the minimum necessary to defeat the Americans in any subsequent war. The Japanese envisaged two separate engagements, first with the U.S. Pacific Fleet and then with the U.S. Atlantic Fleet. It calculated that a 7:5 ratio in the first battle would produce enough of a margin of victory to be able to win the subsequent engagement and so a 3:5 ratio was unacceptable because a 3:5 total fleet size ratio would imply a 6:5 ratio in the first battle. Nevertheless, the director of the delegation, Katō Tomosaburō, preferred to accept the latter to the prospect of an arms race with the United States, as the relative industrial strength of the two nations would cause Japan to lose such an arms race and possibly suffer an economic crisis. At the beginning of the negotiations, the Japanese had only 55% of the capital ships and 18% of the GDP of the Americans.