During World War II, she supported Japanese forces during the invasion of Malaya in December 1941, the Dutch East Indies campaign in early 1942, and the Battle of Midway in June 1942.

Except for brief service in the Aleutian Islands campaign in 1943, she subsequently served on training duties until selected for use as a kaiten manned suicide torpedo carrier in 1945.

[1] For surface running, the submarines were powered by two 3,400-brake-horsepower (2,535 kW) diesel engines, each driving one propeller shaft.

[4] Submarine Division 19 was reduced to the First Reserve in the Kure Naval District on 7 January 1937, and I-56, was decommissioned that day.

[4] By 1 December 1937 she apparently had been recommissioned,[5] and she resumed her direct assignment to the Kure Naval District on 1 January 1938.

[4][5] Tasked with supporting Operation E, the Japanese invasion of British Malaya, I-56 proceeded to her patrol area in the South China Sea northwest of the Anambas Islands.

On 8 December, I-56 unsuccessfully attacked a Royal Netherlands Navy submarine — probably HNLMS K XVII — east of the Malay Peninsula.

[5] On 11 December, she attacked the Norwegian 1,186-gross register ton merchant ship Hai Tung — which was on a voyage from Bangkok, Siam, to Singapore, with a cargo of rice and general supplies — with gunfire, sinking her with the loss of her entire crew of 50 east of British Malaya at 05°08′N 104°32′E / 5.133°N 104.533°E / 5.133; 104.533 (Hai Tung).

[4][5][19] One source states that I-56 rammed one of Kwangtung′s lifeboats and machine-gunned others, and that out of 98 crewmen and 35 military personnel aboard, only 35 survived to be rescued the following day.

[19] Early on the morning of 5 January, she surfaced and opened gunfire on the Dutch 8,169-gross register ton armed merchant ship Tanimbar 40 nautical miles (74 km; 46 mi) southeast of Tjilatjap, damaging the ship,[19] but abandoned the attack and submerged when Tanimbar returned fire.

[5] While submerged about 80 nautical miles (150 km; 92 mi) southwest of Tjilatjap, I-56 hit the Dutch 3,032-gross register ton steamer SS Van Ress — which was on a voyage from Tjilatjap to Emmahaven on Sumatra — in the engine room with one torpedo at around 06:00 on 8 January 1942, killing six of her crewmen.

[4][5] The Royal Netherlands Navy minelayer HNLMS Willem van der Zaan rescued the survivors.

[5] As Patras worked up to her maximum speed of 13 knots (24 km/h; 15 mph), she took several hits in her stern and caught fire.

[5] A Netherlands Naval Aviation Service Dornier Do 24K flying boat arrived on the scene, and I-56 dived to avoid attack by the aircraft.

[5] On 4 February, she attacked the Dutch 979-gross register ton merchant ship Togian — steaming with Convoy JS.1 — with gunfire at the southern entrance to the Sunda Strait.

[5] On 9 March, she sighted the 30-foot (9.1 m) lifeboat Scorpion, which was carrying 12 Allied airmen escaping from Tjilitjap as it fell to Japanese forces and made for Roebourne, Western Australia.

[5] After the commander-in-chief of the Combined Fleet, Admiral Isoroku Yamamoto, ordered Komatsu to interpose his submarines between the retreating Japanese fleet and the opposing United States Navy aircraft carriers,[5] the Japanese submarines, including I-156, began a gradual movement to the north-northwest, moving at 3 knots (5.6 km/h; 3.5 mph) by day and 14 knots (26 km/h; 16 mph) after dark.

[5] At about 04:00 on 5 June 1942, I-156 sighted the U.S. Navy oiler USS Guadalupe (AO-32) escorted by two destroyers 550 nautical miles (1,020 km; 630 mi) east of Midway, but could not get into a firing position.

[5] The only Japanese submarine other than I-168[note 1] to make contact with enemy forces during the battle, I-156 returned to Kwajalein on 20 June 1942.

[5] After resurfaciung, unloading her cargo, and embarking 60 passengers, she got back underway the same day bound for Paramushiro, which she reached on 20 June.

[5] She continued in this role until 1 April 1945, when she was reassigned to Submarine Division 34 for service as a carrier of kaiten manned suicide attack torpedoes.