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 at the time and the first bigger than 15 inches (381 mm).

Mutsu saw more active service than her sister because she was not a flagship and participated in the Battle of the Eastern Solomons in August before returning to Japan in early 1943.

The ships also used a new type of underwater protection system that successfully resisted penetration by 200-kilogram (440 lb) torpedo warheads in full-scale trials.

[7] On 12 June 1917, well before Mutsu was laid down, Hiraga proposed a revised design for the ship that reflected the lessons from the Battle of Jutland that had occurred the previous year, and incorporated advances in boiler technology.

[12] The Nagato class was equipped with a unique heptapodal (seven-legged) mast designed to maximize rigidity for range-finding purposes and survivability under shellfire.

During their 1934–1936 reconstruction, the ships' sterns were lengthened by 7.55 meters (24.8 ft) to improve their speed and their forward superstructures were rebuilt into a pagoda mast.

The turbines were designed to produce a total of 80,000 shaft horsepower (60,000 kW), using steam provided by 21 Kampon water-tube boilers; 15 of these were oil-fired while the remaining half-dozen used fuel oil that was sprayed on the coal to increase its burn rate.

[5] During refits in 1923–25, the fore funnel was rebuilt in a serpentine shape in an unsuccessful effort to prevent smoke interference with the bridge and fire-control systems.

[18] Additional fuel oil was stored in the bottoms of the newly added torpedo bulges, which increased their capacity to 5,560 long tons (5,650 t) and thus their range to 8,560 nmi (15,850 km; 9,850 mi) at 16 knots.

[10] The main armament of the Nagato-class ships consisted of eight 45-caliber 41 cm (16.1 in) 3rd Year Type naval guns, mounted in two pairs of twin-gun, superfiring turrets fore and aft.

[21] The ships' secondary armament of twenty 50-caliber 14 cm (5.5 in) 3rd Year Type naval gun was mounted in casemates on the upper sides of the hull and in the superstructure.

[33] This was the standard Japanese light AA gun during World War II, but it suffered from severe design shortcomings that rendered it a largely ineffective weapon.

About 3 meters (9 ft 10 in) from the side of the hull, this deck, now composed of three layers of HTS, totaling 75 mm (3 in) in thickness, sloped downwards where it met a short horizontal armored (three layers of HTS with a total thickness of 66 mm (3 in)) deck that connected to the main armored belt and the torpedo bulkhead.

[39] The outermost void was designed to allow the explosive force of a torpedo's warhead to dissipate as much as possible while the oil tank was supposed to stop any fragments from penetrating the innermost bulkhead protecting the ship's vital areas.

The Washington Naval Conference convened on 12 November, and the Americans proposed to scrap virtually every capital ship under construction or fitting out among the participating nations.

[50] Upon commissioning, the sister ships were assigned to the 1st Battleship Division, although Nagato became the flagship of Rear Admiral Sōjirō Tochinai, a role she often fulfilled during her career.

They sank the hulk of the obsolete battleship Satsuma on 7 September 1924 during gunnery practice in Tokyo Bay in accordance with the Washington Naval Treaty.

When Japan began the Pacific War on 8 December,[Note 5] the sisters sortied for the Bonin Islands, along with the four ships of Battleship Division 2 and the light carrier Hōshō as distant cover for the fleet attacking Pearl Harbor, and returned six days later.

[13] In June 1942, Mutsu and Nagato were assigned to the Main Body of the 1st Fleet during the Battle of Midway, together with Yamato, Hōshō, the light cruiser Sendai and nine destroyers.

After rendezvousing with the remnants of the Striking Force on 6 June, over half of the survivors from the sunken aircraft carriers of the 1st Air Fleet were transferred to Mutsu and Nagato.

[13] Mutsu was transferred to the Advance Force of the 2nd Fleet on 9 August, and departed Yokosuka two days later to support operations during the Guadalcanal Campaign.

On 20 August, while sailing from Truk to rendezvous with the main body of Vice Admiral Chūichi Nagumo's 3rd Fleet, Mutsu, the heavy cruiser Atago, and escorting destroyers unsuccessfully attempted to locate the escort carrier USS Long Island in response to a flying boat detection of the American ship.

[51] During the Battle of the Eastern Solomons on 27 August, Mutsu, assigned to the Support Force,[56] fired four shells at enemy reconnaissance aircraft during what was her first, and only, action of the war.

Nearby ships were able to rescue 353 survivors from the 1,474 crew members and visitors aboard Mutsu, meaning that 1,121 men were killed in the explosion.

Nagato remained at Lingga until 11 May when she was transferred to Tawitawi on 12 May, and assigned to the 1st Mobile Fleet, under the command of Vice Admiral Jisaburō Ozawa.

[13] Battleship Division 1 rendezvoused with Ozawa's main force on 16 June near the Mariana Islands, and Nagato escorted three aircraft carriers during the Battle of the Philippine Sea.

[13] A coal-burning donkey boiler was installed on the pier for heating and cooking purposes and a converted submarine chaser was moored alongside to provide steam and electricity;[49] her anti-aircraft guns lacked full power and were only partially operational.

[13][49] Nagato was selected to participate as a target ship in Operation Crossroads, a series of U.S. nuclear weapon tests held at Bikini Atoll in mid-1946.

In mid-March, Nagato departed Yokosuka for Eniwetok, but her hull had not been repaired from the underwater damage sustained during the attack on 18 July and she leaked enough that her pumps could not keep up.

[13] Operation Crossroads began with the first blast (Test Able), an air burst on 1 July 1946; Nagato was not close to ground zero and was only lightly damaged.