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War Plan Orange (commonly known as Plan Orange or just Orange) refers to a series of United States Joint Army and Navy Board war plans for dealing with a possible war with Japan during the years between the First and Second World Wars.
Informal studies as early as 1906 covered a number of possibilities, from basing at Gibraltar or Singapore (an idea revived by the British before World War II) to "a quick trans-Atlantic dash" to the Pacific. The plan eventually adopted was conceived by Rear Admiral Raymond P. Rodgers in 1911.
The plan was formally adopted by the Joint Army and Navy Board beginning in 1924. Predating the Rainbow plans, which presumed the assistance of allies, Orange was predicated on the U.S. fighting Japan alone.
As originally conceived, it anticipated a withholding of supplies from the Philippines and other US outposts in the Western Pacific (they were expected to hold out on their own), while the Pacific Fleet marshaled its strength at bases in California, and guarded against attacks on the Panama Canal. After mobilization (the ships maintained only half of their crews in peacetime), the Fleet would sail to the Western Pacific to relieve American forces in Guam and the Philippines. Afterwards, the fleet would sail North for a decisive battle against the Imperial Japanese Navy's Combined Fleet, and then blockade the Japanese home islands. This was in keeping with the theory of Alfred Thayer Mahan, a doctrine to which every major navy subscribed before World War II, in which wars would be decided by engagements between opposing surface fleets (as they had been for over 300 years).
Rodgers' concept was closer to the one ultimately used in the Pacific War: a "leapfrog" campaign to conquer the Marshalls and Carolines (held by Japan before the war); liberation of the Philippines; and blockade. Absent was the "decisive battle" of Mahan, and of Japanese planning.
American war planners failed to appreciate that technological advances in submarines and naval aviation had made Mahan's doctrine obsolete. In particular, they did not understand that aircraft could effectively sink battleships, nor that Japan might put the US battleship force (the Battle Line) out of action at a stroke—as in fact happened during Pearl Harbor.
American plans changed after this attack. Even after major Japanese defeats like Midway, the US favored a methodical "island-hopping" advance, never going far beyond land-based air cover. Meanwhile, blockade was imposed from the very beginning of the war, with the first American submarine, Joe Grenfell's Gudgeon, arriving off Japan on about 31 December.
A number of requirements grew out of Orange, including the specification for a fleet submarine with high speed, long range, and heavy torpedo armament. These coalesced in the submarine Dolphin in 1932 (only to be rejected and returned to with the Gato class in around August 1941). The demand for submarines of this size also drove the development of the notorious Mark XIV torpedo (and its equally notorious Mark VI exploder), under the guidance of Commander Ralph W. Christie. The Navy also spent "several hundred thousand dollars" to develop powerful, compact diesel engines, among them the troublesome Hooven-Owens-Rentschler (HOR), which proved useful for railroads.
The Imperial Japanese Navy developed a counter-plan to allow the US Pacific Fleet to sail across the Pacific while using submarines and carrier attacks to weaken it. The Japanese fleet would then attempt to force a battle against the weakened US fleet in a "decisive battle area", near Japan (see Kantai Kessen), also in line with Mahanian doctrine, which Japan had enthusiastically embraced. It was the basis for Japan's demand for a 70% ratio (10:10:7) at the Washington Naval Conference, which was considered necessary to provide Japan superiority in the "decisive battle area", (taking into account that the US had naval commitments in other theaters, while Japan did not), as well as the US's insistence on 60%, which amounted to parity.
Actual events were very close to the final plan. Carrier battles overshadowed surface action, but the "leapfrog" campaign was very much as anticipated.
The Imperial Japanese Navy, focused with the "decisive battle" doctrine, ignored the vital need for defense against submarines. The German and American submarine campaigns against their opponents' merchant shipping demonstrated the need for ASW. While the Allies took extensive measures to combat the threat of German U-boats, the Japanese failed to counter the American submarines which ultimately choked Japan's industrial production and paralyzed her navy. Japan also notably failed to institute an anti-commerce campaign herself; systematic use of commerce raiders could have made Allied operations much more complex and conquering and holding Japanese-held islands more difficult.