The Second London Naval Treaty was an international treaty signed as a result of the Second London Naval Disarmament Conference held in London, the United Kingdom.
The conference started on 09 December 1935 and treaty was signed by the participating nations on 25 March 1936.
The signatories were France, the United States, and most members of the British Commonwealth: Australia, Canada, India, New Zealand, and the United Kingdom (on behalf of itself and “all parts of the British Empire which are not separate Members of the League of Nations”). Two Commonwealth Dominions declined to sign: South Africa and the Irish Free State, the latter because it had no navy. Japan, a signatory of the First London Naval Treaty and already at war on the Asian mainland, withdrew from the conference on 15 January. Italy also declined to sign the treaty, largely as a result of the controversy over its invasion of Abyssinia (Ethiopia); Italy was under sanctions from the League of Nations.
The conference was intended to limit the growth in naval armaments until its expiration in 1942. The absence of Japan (a very significant naval power) prevented agreement on a ceiling on the numbers of warships. The treaty did limit the maximum size of the signatories’ ships, and the maximum calibre of the guns which they could carry. First of all, capital ships were restricted to a 35,000 long tons (35,562 t) standard displacement and 14-inch (356 mm) guns. However, a so-called “escalator clause” was included at the urging of American negotiators in case any of the countries that had signed the Washington Naval Treaty refused to adhere to this new limit. This provision allowed the signatory countries of the Second London Treaty – France, the United Kingdom and the United States – to raise the limit from 14-inch guns to 16-inch if Japan or Italy still refused to sign after 01 April 1937.
Also submarines could not be larger than 2,000 tons or have any gun armament of greater than 5.1-inches, light cruisers were restricted to 8,000 tons and 6.1-inch (155 mm) or smaller guns and aircraft carriers were restricted to 23,000 tons. Article 25 however gave the right to depart limitations if any other country authorised, constructed or acquired a capital ship, an aircraft carrier, or a submarine exceeding treaty limits, and if such a departure would be necessary for national security. For this reason, in 1938 the treaty parties agreed on a new displacement limit of 45,000 tons for battleships, the ill-fated battlecruiser already having fallen out of favour.
This London Naval Treaty effectively ended on 01 September 1939 with the beginning of World War II. Even during its brief period of supposed effectiveness, its clauses were honoured more in the breach than in the observance. Three classes of “treaty” battleships were built or laid down by the United States: the North Carolina, South Dakota, and Iowa classes. The design of the North Carolina class was initiated before the escalator clause was invoked, Its ships being intended to be armed with, and protected against, 14-inch guns. However, with the invocation of the escalator clause, they were completed with 16-inch guns. The four battleships of the South Dakota class were designed with and protected against 16-inch guns, but maintained a 35,000 ton standard displacement. Design of the Iowa-class began in 1938 and its orders were placed in 1939; with the invocation of the “escalator clause”, the Iowas carried 16-inch guns on a displacement of 45,000 tons.
Article 22 of the 1930 Treaty of London relating to submarine warfare declared international law (the so-called “cruiser rules”) applied to submarines as well as to surface vessels. Also, unarmed merchant vessels which did not demonstrate “persistent refusal to stop…or active resistance to visit or search” could not be sunk without the ships’ crews and passengers being first delivered to “a place of safety” (for which lifeboats did not qualify, except under particular circumstances). The 1936 treaty confirmed Article 22 of the 1930 treaty remained in force, and “all others Powers [were invited] to express their assent to the rules embodied in this Article”. This became known as the London Submarine Protocol, and over thirty-five nations eventually did subscribe to it, including the US, Britain, Germany, and Japan. It was this Protocol which was used at the post war Nuremberg Trial of Karl Dönitz for ordering unrestricted submarine warfare. These regulations did not prohibit arming merchantmen, but according to Dönitz, arming them, or having them report contact with submarines (or raiders), made them de facto naval auxiliaries and removed the protection of the cruiser rules. This made restrictions on submarines effectively moot.