The period from 11 November 1918 until the Japanese attack on Pearl Harbor, HI on 7 December 1941 was a period of exploration for the submarine branches of all navies. Despite their successes in World War I, submarines were still relatively new to naval warfare, and submariners were still trying to find the proper combat role for their craft. Many influential naval leaders, such as America's Fleet Admiral Chester W. Nimitz, began their careers in "the boats".
During this period, as technology improved, submarines were continually setting, then breaking, new records for speed, range, and endurance.
Experiments were conducted by several nations with sea-planes carried aboard submarines and large guns mounted on submarine decks. That the experiments were conducted at all attests to the zeal of submariners around the world. That few submarines were involved in serious accidents during these experiments attests to their skills. This spirit and these skills made these accidents all the harder to bear, for this period of submarine operations was not easy, and submariners of all nations would pay the ultimate price.
| Submarine | Nationality | Date | Type of Accident |
| G.11 | British | 22 November 1918 | Grounding |
| O-12 (SS-73) | American | 27 March 1919 | Diving Accident |
| H.41 | British | 18 October 1919 | Allision |
| USS H-1 (SS-28) | American | 12 March 1920 | Grounding |
| USS S-5 (SS-110) | American | 1 August 1920 | Foundered |
| K.5 | British | 20 January 1921 | Foundered |
| K.15 | British | 25 June 1921 | Flooding |
| USS R-6 (SS-83) | American | 26 September 1921 | Flooding |
| USS S-48 (SS-159) | American | 07 December 1921 | Foundered |
| Fieldmarshal Graf Sheremetev | Russian | 1922 | Flooding |
| H.42 | British | 23 March 1922 | Collision |
| L.9 | British | 18 January 1923 | Typhoon |
| USS S-38 (SS-143) | American | 17 July 1923 | Flooding |
| USS S-37 (SS-142) | American | 10 October 1923 | Explosion |
| USS O-5 (SS-66) | American | 29 October 1923 | Collision |
| L.24 | British | 10 January 1924 | Collision |
| USS S-48 (SS-159) | American | 29 January 1925 | Grounding |
| USS S-51 (SS-162) | American | 23 September 1925 | Collision |
| M.1 | British | 12 November 1925 | Collision |
| S-49 (SS-160) | American | 20 April 1926 | Battery Explosion & Fire |
| H.29 | British | 09 August 1926 | Flooding |
| USS S-4 (SS-109) | American | 17 December 1927 | Collision |
| H.47 & L.12 | British | 09 July 1929 | Collision |
| Rabochiy | Russian | 22 May 1931 | Collision |
| Poseidon | British | 09 June 1931 | Collision |
| L-55 | Russian | 24 October 1931 | Collision |
| M.2 | British | 26 January 1932 | Unknown |
| S-34 (SS-139) | American | 11 January 1934 | Signal Cartridge Explosion |
| B-3 (ex-Bolshevik) | Russian | 25 July 1935 | Collision |
| U-18 | Germany | 20 November 1936 | Collision |
| USS Squalus (SS-192) | American | 23 May 1939 | Foundered |
| Thetis | British | 01 June 1939 | Foundered |
| USS O-9 (SS-70) | American | 20 June 1941 | Foundered |
Continue to next section, World War II
Return to Peacetime Submarine Losses