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The U.S. had the largest and most powerful submarine force of all the Allied countries in the Pacific at the outbreak of war.prize rules set out in the London Naval Treaty, to which the U.S. was a signatory. The U.S. Navy built large submarines which boasted long range, a relatively fast cruising speed and a heavy armament of torpedoes. United States submarines were better suited for long patrols in the tropics than those of the other major powers due to amenities such as air conditioning (which German U-boats, for instance, lacked) and water distilleries. The submarines' commanders and crewmen were considered elite and enjoyed a strong esprit. On 7 December 1941, the USN had 55 fleet- and 18 medium-sized submarines (S-boats) in the Pacific, 38 submarines elsewhere, and 73 under construction. (By war's end, the U.S. had completed 228 submarines.)
Pre-war U.S. Navy doctrine—like that of all major navies—specified that the main role of submarines was to support the surface fleet by conducting reconnaissance and attacking large enemy warships. Merchant ships were regarded as secondary targets, and the circumstances in which they could be attacked were greatly limited by
While Britain stationed a force of submarines in the Far East prior to the outbreak of war, no boats were available in December 1941. The British had 15 modern submarines in the Far East in September 1939. These submarines formed part of the China Station and were organised into the 4th Flotilla. Although the number of British submarines in the Far East increased in early 1940 when the 8th Flotilla arrived at Ceylon, both flotillas and all their submarines were withdrawn in mid-1940 to reinforce the Mediterranean Fleet.
The Netherlands also maintained a submarine force in the Far East in order to protect the Netherlands East Indies (NEI). In December 1941, this force comprised 15 boats based at Surabaya, most of which were obsolete.