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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
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
flotillas and all their submarines were withdrawn in mid-1940 to reinforce the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.