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The Washington Naval Treaty, also known as the Five-Power Treaty, and including the "Four-Power Treaty" and the "Nine-power Treaty" was a treaty among the major nations that had won World War I, which agreed to prevent an arms race by limiting naval construction. It was negotiated at the Washington Naval Conference, held in Washington, D.C., from November 1921 to February 1922, and it was signed by the governments of the United Kingdom, the United States, Japan, France, and Italy. 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.
The naval treaty was concluded on February 6, 1922. Ratifications of that treaty were exchanged in Washington on August 17, 1923, and it was registered in League of Nations Treaty Series on April 16, 1924.
Subsequent to the treaty, other naval arms limitation conferences sought to increase limitations of warship building. The terms of the Washington 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 renounced the treaties, making naval arms limitation an increasingly untenable position for the other signatories.
Immediately after World War I, the United Kingdom had the world's largest and most powerful navy, followed by the United States and more distantly by Japan. The three nations had been allied for World War I, but a naval arms race seemed likely for the next few years. The arms race began in the United States. President Woodrow Wilson's administration 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. At the time, it was engaged in building six battleships and six battlecruisers.
In response, the Japanese parliament finally authorised construction of warships to enable the Japanese Navy to reach its target of an "eight-eight" fleet programme, with eight modern battleships and eight battlecruisers. The Japanese started work on four battleships and four battlecruisers, all much larger and more powerful than those of the classes preceding. 
The 1921 British Naval Estimates planned four battleships and four battlecruisers, with another four battleships to follow the subsequent year.
The U.S. public was largely unwelcoming of the new "arms race". The United States Congress disapproved of Wilson's 1919 naval expansion plan, and during the 1920 presidential election campaign, politics returned to the non-interventionalism of the prewar era, with little appetite for continued naval expansion. Britain could also ill afford any resumption of battleship construction, given the exorbitant price of naval construction.
In late 1921, the US government became aware that Britain was planning a conference to discuss the strategic situation in the Pacific and Far East. To forestall the conference and satisfy domestic pressure for a global disarmament conference, the Harding administration called the Washington Naval Conference during November 1921.
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 shortened the conference while helping ensure his proposals were largely adopted. He subsequently proposed the following:
The proposals regarding capital ships were largely accepted by the British delegation, but they were controversial with the public. It would no longer be possible for Britain to have adequate fleets in the North Sea, the Mediterranean, and the Far East simultaneously. The facts provoked outrage from parts of the Royal Navy. Nevertheless, there was huge pressure on Britain to agree. The risk of war with the United States was increasingly regarded as merely theoretical, as there were very few policy differences between the two countries. Naval spending was unpopular in both Britain and its dominions. Furthermore, Britain was implementing major decreases of its budget because of the economic crisis created by the end of the war.
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 United States 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 a big enough margin of victory to be able to win the subsequent engagement so a 5:3 ratio, or 60%, was unacceptable. Nevertheless, the director of the delegation, Katō Tomosaburō, preferred accepting a 60% ratio to the prospect of an arms race with the United States, as the relative industrial output of the two nations would cause Japan to lose such an arms race and might cause it an economic crisis. Going into the negotiations the Japanese had only 55% as many capital ships as the Americans and 18% of the GDP.
His opinion was opposed strongly by Katō Kanji, the president of the Naval Staff College, who acted as his chief naval aide at the delegation and represented the influential "big navy" school of thought. It held that in the event of war the United States would be able to build indefinitely more warships, given its huge industrial power, and Japan thus needed to prepare as thoroughly as possible for the inevitable conflict with America. Katō Tomosaburō was finally able to persuade the Japanese high command to accept the Hughes proposals, but the outcome of the treaty was a cause of controversy in the Japanese navy for years to come.
The French delegation initially responded angrily to the idea of reducing its capital ships tonnage to 175,000 tons and demanded 350,000, slightly above Japan. In the end, concessions regarding cruisers and submarines helped persuade the French to agree to the limit on capital ships. Another issue that was seen as critical by the French representatives was Italy's request of substantial parity, which was seen as unsubstantiated; however, pressure from the American and British delegations led them to give in. That was seen as a great success in Italy although parity would never actually be reached.
There was much discussion about the inclusion or exclusion of individual warships. In particular, the Japanese delegation was keen to retain their newest battleship Mutsu, which had been funded with great public enthusiasm, including donations from schoolchildren. That resulted in provisions to allow the United States and Britain to construct equivalent ships.
Hughes proposed to limit secondary ships (cruisers and destroyers) in the same proportions as capital ships. However, that was unacceptable to both the British and the French. The British counterproposal, in which the British would be entitled to 450,000 tons of cruisers in consideration of its imperial commitments but the United States and Japan only 300,000 and 250,000 respectively, proved equally contentious. Thus, the idea of limiting total cruiser tonnage or numbers was rejected entirely.
Instead, the British suggested a qualitative limit of future cruiser construction. The limit proposed, of a 10,000 ton maximum displacement and 8-inch calibre guns, was intended to allow the British to retain the Hawkins class, then being constructed. That coincided with US requirements for cruisers for Pacific operations and also with Japanese plans for the Furutaka class. The suggestion was adopted with little debate.
A major British demand during the negotiations was the complete abolition of the submarine, which had proved so effective against them in the war. However, that proved impossible, particularly as a result of French opposition; they demanded an allowance of 90,000 tons of submarines and so the conference ended without an agreement on restricting submarines.
Article XIX of the Treaty also prohibited Britain, Japan, and the United States from constructing any fortifications or naval bases in the Pacific Ocean. That was a significant victory for Japan, as fortified British or American bases would pose a serious problem for the Japanese in the event of any future war. The provision of the treaty essentially guaranteed that Japan would be the dominant power in the Western Pacific Ocean and was crucial in gaining Japanese acceptance of the limits on capital ship construction.
|Country||Capital ships||Aircraft carriers|
|British Empire||525,000 tons
|United States||525,000 tons
|Empire of Japan||315,000 tons
The treaty strictly limited both the tonnage and construction of capital ships and aircraft carriers and included limits of the size of individual ships.
The tonnage limits defined by Articles IV and VII (tabulated) gave a strength ratio of approximately 5:5:3:1.75:1.75 for Britain, the United States, Japan, Italy, and France.
The qualitative limits of each type of ship were as follows:
The treaty also detailed by Chapter II the individual ships to be retained by each navy, including the allowance for the United States to complete two further ships of the Colorado class and for Britain to complete two new ships in accordance with the treaty limits.
Chapter II, part 2, detailed what was to be done to render a ship ineffective for military use. In addition to sinking or scrapping, a limited number of ships could be converted as target ships or training vessels if their armament, armour and other combat-essential parts were removed completely. Some could also be converted into aircraft carriers.
Part 3, Section II specified the ships to be scrapped to comply with the treaty and when the remaining ships could be replaced. In all, the United States had to scrap 30 existing or planned capital ships, Britain 23 and Japan 17.
The treaty marked the end of a long period of increases of battleship construction. Many ships then being constructed were scrapped or converted into aircraft carriers. Treaty limits were respected and then extended by the London Naval Treaty of 1930. It was not until the mid-1930s that navies began to build battleships once again, and power and size of new battleships began to increase once again. The Second London Naval Treaty in 1936 sought to extend the Washington Treaty limits until 1942, but in the absence of Japan or Italy, it was largely ineffective.
There were fewer effects on cruiser building. While the treaty specified 10,000 tons and 8-inch guns as the maximum size of a cruiser, that was also the minimum size cruiser that any navy was willing to build. The treaty began a building competition of 8-inch, 10,000 ton "treaty cruisers", which gave further cause for concern. Subsequent naval treaties sought to address this, by limiting cruiser, destroyer and submarine tonnage.
Unofficial terms of the treaty included the end of the Anglo-Japanese Alliance by Britain. It was not part of the Washington Treaty in any way, but the American delegates had made it clear they would not agree to the treaty unless Britain ended its alliance with the Japanese. 
The naval treaty had a profound effect on the Japanese. With superior American and British industrial power, a long war would very likely end in a Japanese defeat. Thus, gaining parity on the strategic level was not economically possible.
Many Japanese considered the 5:5:3 ratio of ships as another way of being snubbed by the West, but it can be argued that the Japanese had a greater concentration of force than the U.S Navy or the Royal Navy. It also contributed to controversy in high ranks of the Imperial Japanese Navy between the Treaty Faction officers and their Fleet Faction opponents, who were also allied with the ultranationalists of the Japanese army and other parts of the Japanese government. For the Treaty Faction, the treaty was one of the factors that contributed to the deterioration of the relationship between the United States and Japanese governments. Some have also argued that the treaty was one major factor in prompting Japanese expansionism by the Fleet Faction during the early 1930s. The perception of unfairness resulted in Japan's renunciation of the Naval Limitation Treaties in 1936.
Isoroku Yamamoto, who later masterminded the Attack on Pearl Harbor, argued that Japan should remain in the treaty. His opinion was more complex, however, in that he believed the United States could outproduce Japan by a greater factor than the 5:3 ratio because of the huge US production advantage of which he was an expert since he had served with the Japanese Embassy in Washington. "Anyone who has seen the auto factories in Detroit and the oil-fields in Texas", he commented after the signing of the treaty, "knows that Japan lacks the power for a naval race with America." He later added, "The ratio works very well for Japan – it is a treaty to restrict the other parties." He believed that other methods than a spree of construction would be needed to even the odds, which may have contributed to his advocacy of the plan to attack Pearl Harbor. However, he did not have sufficient influence at Navy headquarters or in the government.
On December 29, 1934, the Japanese government gave formal notice that it intended to terminate the treaty. Its provisions remained in force until the end of 1936 and were not renewed. Japan effectively ignored the treaty in 1936.
What was unknown to the participants of the Conference was that the American "Black Chamber" (the Cypher Bureau, a US intelligence service), commanded by Herbert Yardley, was spying on the delegations' communications with their home capitals. In particular, Japanese communications were penetrated thoroughly, and American negotiators were able to get the minimum possible deal the Japanese had indicated they would ever accept, less than which they would renounce the Conference. As this ratio value was unpopular with much of the Imperial Japanese Navy and with the increasingly active and important ultranationalist groups, the value that the Japanese government accepted was the cause of much suspicion and accusation among Japanese politicians and naval officers.
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