The Nagato-class battleships were a pair of dreadnought battleships built for the Imperial Japanese Navy (IJN) towards the end of World War I, although they were not completed until after the war. The last of Japan's pre-Treaty capital ships, they were the first class to carry 41 cm (16.1 in) guns, the largest afloat and the first bigger than 15 inches (381 mm). Nagato, the lead ship of the class, frequently served as a flagship. Both ships carried supplies for the survivors of the Great Kantō earthquake in 1923. They were modernized in 1933–1936 with improvements to their armor and machinery and a rebuilt superstructure in the pagoda mast style. Nagato and her sister ship Mutsu briefly participated in the Second Sino-Japanese War in 1937 and Nagato was the flagship of Admiral Isoroku Yamamoto during the attack on Pearl Harbor on 7 December 1941 that began the Pacific War.

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.