A treaty battleship was a battleship built in the 1920s or 1930s under the terms of one of a number of international treaties governing warship construction. Many of these ships played an active role in the Second World War, but few survived long after it.
In the Washington Naval Treaty of 1922, the world's five naval powers agreed to abide by strict restrictions on the construction of battleships and battlecruisers, in order to prevent an arms race in naval construction such as preceded the Great War. The Treaty limited the number of capital ships possessed by each signatory, and also the total tonnage of each navy's battleships. New ships could only be constructed to replace the surviving ships as they retired after 20 years' service. Furthermore, any new ship would be limited to guns of 16-inch caliber and a displacement of 35,000 tons.
The Washington Treaty limits were extended and modified by the London Naval Treaty of 1930 and the Second London Naval Treaty of 1936. During the 1930s, however, the effectiveness of these agreements broke down, as some signatory powers (in particular Japan) withdrew from the treaty arrangements and others only paid lip service to them. By 1938, Britain and the USA had both invoked an 'escalator clause' in the Second London Treaty which allowed battleships of up to 45,000 tons displacement, and the Treaty was effectively defunct.
The strict limits on displacement forced the designers of battleships to make compromises which they might have wished to avoid given the choice. The 1920s and 1930s saw a number of innovations in battleship design, particularly in engines, underwater protection, and aircraft.
The Washington Naval Treaty was signed in 1922 by Britain, France, the United States, Japan and Italy, as the five powers which had any significant numbers of modern dreadnought battleships and battlecruisers. The Treaty was aimed at preventing an expensive arms race, principally between Britain, the USA and Japan. The Treaty established a definition of a capital ship, which was any ship with a displacement of 10,000 tons or more, or with guns above 8 in (203 mm) calibre, apart from an aircraft carrier. Carriers were specifically constrained from having guns above 8 in calibre, in order to prevent confusion between the two types. Each signatory agreed to limit its total number of capital ships, and its total tonnage of capital ships. These measures meant that several classes of battleships and battlecruisers which were planned, or had even been begun, were scrapped or cancelled.
In addition, each new ship was limited in size to 35,000 tons displacement, and to guns of 16 in (410 mm) calibre. Only one vessel already finished, the British battlecruiser HMS Hood, exceeded these limits; however, many of the new ships planned or being built were significantly larger. The Treaty permitted the improvement of existing warships, but limited the resulting increase in displacement at 3,000 tons.
The Treaty also introduced a 'building holiday'. In general, a new ship could only be begun if one of the ships allowed by the Treaty had been in service for 20 years. This meant that for most signatory powers, no new ships could be begun until the 1930s. An exception was made for Britain; the Royal Navy had no existing ships with 16-inch guns, while both the U.S. Navy and Japan had 16-inch ships already in commission which would be allowed to continue under the terms of the Treaty.
These first British treaty battleships became the Nelson class, which were begun in 1922 and launched in 1925. The Nelson class solved the problem posed by the new weight restriction by placing all the heavy guns forward of the superstructure in three triple turrets, hence saving weight on the armour around them.
These limits were reiterated by the London Naval Treaty of 1930, and the Second London Naval Treaty of 1936 further limited guns to 14-inch calibre. The Second London Treaty contained a clause which allowed construction of battleships with 16-inch guns if any of the signatories of the Washington Treaty failed to ratify the new one. It contained an additional clause which allowed displacement restrictions to be relaxed if non-signatories built vessels more powerful than the treaty allowed.
Virtually all battleships built subsequently obeyed the treaty limits. The Washington Naval Treaty was signed by the USA, UK, Japan, France and Italy - all the principal naval powers. At various stages Italy and France opted out of further negotiations; however, their economic resources did not permit the development of super-battleships. Germany, while not permitted any battleships by the Treaty of Versailles, developed one in the 1930s; this was legitimised by the Anglo-German Naval Agreement, which placed Germany under the same legal limits as Britain. Only Japan, which opted out of the Treaty system in 1934,[a] actually built mammoth treaty-busting battleships - the Yamato class. The collapse of the treaty system led to the wartime construction of "post-treaty" battleships which exceeded the limitations: the German Bismarck class, the U.S. Iowa class and the British HMS Vanguard and (never completed) Lion class. A number of designs, never finished, shattered the treaty limits; the German H-class were scrapped on the outbreak of war, while the U.S. Montana class were canceled before being laid down.
Treaty battleships were technically superior to their predecessors. Naval technology developed in the 1920s and 1930s provided improved steel, better guns, more efficient engines, and more effective protection against torpedoes. The displacement limit also encouraged naval designers to think creatively about minimising displacement, meaning that the treaty battleships significantly increased their performance.