[2] Compared to the German submarine, they were larger — 10 feet (3 m) longer, and displacing 220 more tons on the surface and 300 more tons submerged — and had a longer range both on the surface — 970 nautical miles (1,800 km; 1,120 mi) farther at 8 knots (15 km/h; 9.2 mph) — and submerged — 5 nautical miles (9.3 km; 5.8 mi) farther at 4.5 knots (8.3 km/h; 5.2 mph).

[4] I-21′s tour of duty in Submarine Division 9 came to an end on 1 December 1930, when she was assigned directly to the Kure Naval District.

[3][4] After her sister ship I-24 suffered damage to her main ballast tanks while I-23 and I-24 were conducting deep diving trials on 25 May 1935,[5] Submarine Division 13 was assigned to the Kure Defense Squadron in the Kure Naval District on 15 November 1935,[4] and that day I-21 was decommissioned so that her main ballast tanks could be reinforced.

[3] In an effort to reduce international tensions over the conflict in China, Japan withdrew its submarines from Chinese waters in December 1938.

[3] As the Imperial Japanese Navy began to deploy in preparation for the impending conflict in the Pacific, I-121 departed Yokosuka in November 1941 bound for Samah on Hainan Island in China.

[4] On 7 December 1941, the last day in East Asia before the attack, I-121 laid forty-two Type 88 Mark 1 mines northeast of Singapore in the vicinity of 00°40′N 115°00′E / 0.667°N 115.000°E / 0.667; 115.000.

[4] In East Asia, the Pacific campaign of World War II began on 8 December 1941, and Japanese forces invaded British Malaya to the north of Singapore that day.

[4] At 05:30 on 11 January 1942, she unsuccessfully attacked an unidentified Allied destroyer in the Timor Sea off the coast of Australia's Northern Territory southwest of the Tiwi Islands.

[4] By 04:55 on 12 January 1942, she had laid 39 mines in the Clarence Strait between Melville Island and the mainland of Australia, after which she returned to her patrol area in the Timor Sea.

[4] She pursued an unidentified Allied merchant ship escorted by a patrol vessel in the Timor Sea west of Wetar on 18 January 1942 and fired three torpedoes at it.

[4] She then returned to the Clarence Strait, where on 21 January 1942 three Allied patrol vessels attacked her, dropped 42 depth charges, and damaged two of her fuel tanks.

[4] In company with I-122, I-121 departed Davao on 9 February 1942 to begin her third war patrol, with a primary mission of supporting an impending air attack on Port Darwin by planes from the aircraft carriers of Vice Admiral Chuichi Nagumo′s Mobile Force and land-based aircraft from both Ambon and Kendari, Celebes.

[4] Her specific tasks included a reconnaissance of Port Darwin prior to the attack and the transmission from the Arafura Sea off northern Australia of weather reports to Nagumo's ships, which would be approaching from Palau.

After patrolling 300 nautical miles (560 km; 350 mi) northeast of Morotai, she parted company with I-122 — which independently headed southeast — on 13 February 1942 and proceeded south on her own.

[4] By sunset on 16 February 1942, I-121 had reached her patrol area 50 nautical miles (93 km; 58 mi) northwest of Darwin, Australia.

[4] After dark, she surfaced again to finish recharging her batteries, and at 20:30 sent a weather report to Nagumo's ships, which were underway from Palau.

[4] At 09:57 that morning, 188 aircraft from Nagumo's carriers began an attack on Darwin's harbor, airfields, and urban center, followed by a raid by 55 Japanese land-based bombers.

[4] After replenishing and refueling, I-121 departed Staring Bay in company with I-122 on 10 March 1942 — the same day they were subordinated directly to the headquarters of the Combined Fleet — and headed for Japan.

[4] With her repairs completed, I-121 departed Kure bound for Kwajalein on 8 May 1942, deploying to participate in Operation MI, the planned Japanese invasion of Midway Atoll.

[4] She stopped at Kwajalein from 19 to 21 May 1942, when she got underway to support a preliminary phase of the Midway operation, Operation K-2, which called for I-121 and I-123 to refuel two Kawanishi H8K (Allied reporting name "Emily") flying boats at the French Frigate Shoals in the Northwest Hawaiian Islands so that the two aircraft could conduct a reconnaissance flight over Pearl Harbor,[4] while I-122 patrolled south of Pearl Harbor to rescue the crews of the aircraft if they were shot down and the submarine I-171 operated east of the French Frigate Shoals to provide a radio beacon for the planes.

[4] While southwest of Lisianski Island en route to her new patrol area on 5 June 1942, I-121 sighted the submarine USS Dolphin (SS-169) heading northeast on the surface.

[4] At 14:59 local time Dolphin sighted I-121′s periscope 2,000 yards (1,830 m) dead astern of her and moved out of range before I-121 could achieve a firing solution against her.

[4] The U.S. submarine USS S-38 (SS-143) sighted I-121 on the surface 30 nautical miles (56 km; 35 mi) south of Cape St. George on New Ireland on 8 August 1942 but was unable to attack her.

[4] In her new patrol area, I-121 unsuccessfully attacked U.S. Navy Task Force 16 50 nautical miles (93 km; 58 mi) southeast of San Cristobal on 22 August 1941.

[4] I-121 was on the surface recharging her batteries on 27 August 1942 when two SBD Dauntless dive bombers from the aircraft carrier USS Wasp (CV-7) attacked her at 06:30.

[4] After arriving in Japan, the aging I-121 and I-122, by then considered obsolescent, were withdrawn from combat and assigned duty as training ships at Kure Naval Base.

[4] Between 23 and 25 February 1945, the two submarines took part in the second stage of the experiments, again in the Seto Inland Sea, with the same camouflage pattern extended to their decks.