The Amagi class (天城型, Amagi-gata) was a series of four battlecruisers planned for the Imperial Japanese Navy (IJN) as part of the Eight-eight fleet in the early 1920s. The ships were to be named Amagi, Akagi, Atago, and Takao. The Amagi design was essentially a lengthened version of the Tosa-class battleship, but with a thinner armored belt and deck, a more powerful propulsion system, and a modified secondary armament arrangement. They were to have carried the same main battery of ten 41 cm (16.1 in) guns and been capable of a top speed of 30 knots (56 km/h; 35 mph).

Limitations imposed by the 1922 Washington Naval Treaty prevented the class from being completed as designed. However, the treaty had a limited allowance for hulls already under construction to be converted into aircraft carriers. Amagi and Akagi were both intended for conversion, but an earthquake damaged the hull of Amagi so extensively that the ship was scrapped. Akagi was reconstructed as an aircraft carrier and served with distinction as part of the Kido Butai during the Second World War, participating in the Japanese attack on Pearl Harbor before being sunk at the Battle of Midway.

Experiences in the Russo-Japanese War convinced naval war planners that more fast capital ships were needed, so on 4 April 1907, the Imperial Defence Council approved an "Eight-eight" policy. This plan originally called for a fleet of eight battleships and eight armored cruisers that would all be under ten years old (later changed to eight battlecruisers and reduced to eight years old). However, the advent of the dreadnought battleship crippled this plan at the beginning; given Japan's weak and underdeveloped economy and the enormous strain that had been put on it during the Russo-Japanese War (Japan emerged from the war victorious, but bankrupt),^[4] the launch of HMS Dreadnought was a "disaster" for Japan.^[5]

In 1907, Japan was halfway to the eight-eight, with two newly delivered battleships (the Katori class) in the fleet and two more (the Satsuma class) and four armored cruisers authorized or under construction. In addition, three more battleships and four armored cruisers had been authorized, though not funded. However, naval technology was changing; older battleships, including all of Japan's battleships in commission or under construction,^{[A 2]} were quickly rendered obsolete with the commissioning of HMS Dreadnought (hence the terms dreadnought and pre-dreadnought), and armored cruisers were seemingly useless in the face of the new battlecruisers being laid down by Great Britain and Germany. The IJN recognized this, and proposed in 1909 that two battlecruisers be ordered from British plans, with one to be built in Great Britain and one to be built at home. These two ships became the Kongō class. Another pair of Kongos were later built in Japan.^[5]

In 1910, there was still authorization for one battleship and four armored cruisers. This battleship, a more heavily armored version of the Kongō-class battlecruisers, became Japan's first super-dreadnought, Fusō. With these ships, Japan appeared to be getting closer to the eight-eight goal; however, these new ships represented a "new level of naval strength" for the IJN, and they made all previous Japanese capital ships obsolete. This meant that any naval planner aiming for an eight-eight fleet would have to call for seven more battleships and four more battlecruisers^[5] at a time when Japan was trying to weather a worldwide economic depression.^[4]

After proposals from the IJN in 1911 and 1912 for massive shipbuilding programs, the Cabinet compromised down to a "four-four" plan; under this, three new battleships (the other Fusō-class ship and the two Ise-class ships) and no new battlecruisers were authorized.^[6] The Navy did not agree, and instead called for an "eight-four" fleet, while the Imperial Defence Council called for the original eight-eight. The Cabinet relented, and by July 1914, it was decided to aim first for an eight-four fleet, followed by the eight-eight fleet. The eight-four plan was presented to the Diet of Japan in 1915; it aimed to have the eight battleships and four battlecruisers by 1923 with the building of two Nagato-class and two Tosa-class battleships. The problem with this was that the old plan intended all of the ships of the eight-eight fleet to be under eight years old; by the time these new ships were completed, Fusō and the first two Kongō ships would be past their replacement age.^[7]

The plan was approved in 1917, along with funding for two battlecruisers which became the Amagi class. In late 1917, the Navy proposed to expand the eight-four plan by adding two more battlecruisers; this was approved, and two more Amagi-class ships were ordered. However, having eight 41 cm (16 in) gun ships (four battleships and four battlecruisers) on order put an enormous financial strain on Japan, which was spending about a third of its national budget on the Navy. The massive size and scale of its building program was rapidly driving up the cost of naval construction and armament.^[7]

Akagi was the first ship of the class to be laid down; construction began on 6 December 1920 at the naval yard in Kure. Amagi followed ten days later at the Yokosuka naval yard. The projected completion dates for the first pair of ships were December and November 1923, respectively. Atago was laid down in Kobe at the Kawasaki shipyard on 22 November 1921, and was projected to be finished in December 1924. Takao, the fourth and final ship of the class, was laid down at the Mitsubishi shipyard in Nagasaki on 19 December 1921, and was also projected to be completed in December 1924.^[1] The ships were named after several mountains: Amagi, Akagi, Atago, and Takao.^{[A 3]} Takao was initially to have been named Ashitaka after Mount Ashitaka.^[8]

The Washington Naval Treaty, signed in February 1922, greatly reduced the tonnage allowed for capital ships in the signatory nations. The treaty also instituted a moratorium on new warship construction; battlecruisers canceled under this included one class each from Japan, the United States, and Great Britain: the Amagi class, the Lexington class and the G3 class, respectively.^[9] The treaty did allow for battleship and battlecruiser hulls currently under construction to be converted into aircraft carriers, but only if these new carriers were kept under a 27,000-ton limit. Considering that the Amagi class were designed to displace 47,000 t (46,000 long tons; 52,000 short tons) at full load in their battlecruiser configuration,^[1] this would have been a rather difficult displacement to obtain. However, the Americans also had the same problem when designing a conversion of their Lexington class, so an exception, spearheaded by US Assistant Secretary of the Navy Theodore Roosevelt Jr., was added to the treaty that gave the five signatories the option of converting up to two capital ships that were under construction to 33,000-ton aircraft carriers.^[10][11] This resulted in the United States and Japan quickly reordering two ships each. Japan chose Amagi and Akagi, the two ships nearest to completion, for conversion.^[1] Their guns were turned over to the Imperial Japanese Army for use as coastal artillery; three of their main gun turrets were installed in Tokyo Bay, at Busan, Korea, and on Iki Island in the Strait of Tsushima. The rest of their guns were placed in reserve and scrapped in 1943.^[12]

The September 1923 Great Kantō earthquake in Tokyo caused significant stress damage to the hull of Amagi. The structure was too heavily damaged to be usable, and conversion work was abandoned. Amagi was stricken from the navy list and sold for scrapping, which began on 14 April 1924. The other two ships, Atago and Takao, were officially canceled two years later (31 July 1924) and were broken up for scrap in their slipways.^[1] The incomplete Tosa-class battleship Kaga, on which work had stopped on 5 February 1922, was reordered as a carrier to replace Amagi.^[13]