Japanese submarine I-51
Following World War I, the Imperial Japanese Navy General Staff began to re-consider submarine warfare as an element of fleet strategy. Before the war, the Japanese Navy regarded submarines as useful only for short-range coastal point defense. However, based on the success of the Imperial German Navy in deployment of long-range cruiser-submarines for commerce raiding Japanese strategists came to realize possibilities for using the weapon for long range reconnaissance, and in a war of attrition against an enemy fleet approaching Japan. Procurement for a large, long-range Japanese submarine was authorized in fiscal 1918 under the Eight-six fleet program, under the designation S22.
"Project S22" was based on the latest Royal Navy design, the British K class submarine. Japanese ties to Great Britain via the Anglo-Japanese Alliance were still strong. "Project S22" was laid down at Kure Naval Arsenal on 6 April 1921, launched on 29 November 1921 and completed on 20 June 1924.
Allied submarines in the Pacific War
During the war, submarines of the United States Navy were responsible for 55% of Japan's merchant marine losses; other Allied navies added to the toll. The war against shipping was the single most decisive factor in the collapse of the Japanese economy. Allied submarines also sank a large number of IJA troop transports, killing many thousands of Japanese soldiers and hampering the deployment of IJA reinforcements during the battles on the Pacific islands. They also conducted reconnaissance patrols, landed special forces and guerrilla troops and performed search and rescue tasks. The majority of the submarines involved were from the U.S. Navy, with the British Royal Navy committing the second largest number of boats and the Royal Netherlands Navy contributing smaller numbers of boats.