Leapfrogging (also called "islandhopping") was a military strategy employed by the Allies in the Pacific War against Japan and the Axis powers during World War II. The idea was to bypass heavily fortified Japanese positions and instead concentrate the limited Allied resources on strategically important islands that were not well defended but capable of supporting the drive to the main islands of Japan.
By the late 19th century, the U.S. had several interests in the western Pacific to defend; namely, access to the Chinese market, and colonies – the Philippines and Guam – which the U.S. had gained as a result of the Spanish–American War (1898). After Japan's victories in the Sino-Japanese War (1894–95) and in the Russo-Japanese War of 1904, the U.S. began to regard Japan as a potential threat to its interests in the western Pacific. This antagonism was intensified by Japan's objections to an attempt to annex Hawaii to the U.S. (1893) and by Japan's objections to discrimination against Japanese immigrants both in Hawaii (1897) – on this occasion, Japan sent the cruiser Naniwa to Honolulu, Hawaii – and in California (1906, 1913). As a result, the U.S. Navy began to draft, as early as 1897, war plans against Japan, which were eventually code-named "War Plan Orange". The war plan of 1911, which was drafted under Rear Admiral Raymond P. Rodgers, included an island-hopping strategy for approaching Japan.
After World War I, the Versailles Treaty gave Japan a mandate over former German colonies in the western Pacific; specifically, the Mariana, Marshall, and Caroline Islands. If these islands were fortified, Japan could, in principle, deny the U.S. access to its interests in the western Pacific. Therefore, in 1921, Lieutenant Colonel Earl Hancock Ellis of the U.S. Marine Corps drafted "Plan 712, Advanced Base Operations in Micronesia," a plan for war against Japan which updated War Plan Orange by incorporating modern military technology (submarines, aircraft, etc.) and which again included an island-hopping strategy. Shortly afterwards, a British-American reporter on naval affairs, Hector Charles Bywater, publicized the prospect of a Japanese-American war in his books Seapower in the Pacific (1923) and The Great Pacific War (1925), which detailed an island-hopping strategy. The books were read not only by Americans but by senior officers of the Japanese Imperial Navy, who used "island-hopping" in their successful southeast Asia offensives in 1941 and 1942.
Rationale and use 
This strategy was possible in part because the Allies used submarine and air attacks to blockade and isolate Japanese bases, weakening their garrisons and reducing the Japanese ability to resupply and reinforce them. Thus troops on islands which had been bypassed, such as the major base at Rabaul, were useless to the Japanese war effort and left to "wither on the vine." General Douglas MacArthur greatly supported this strategy in his effort to regain the Philippines. This strategy began to be implemented in late 1943 in Operation Cartwheel. While MacArthur claimed to have invented the strategy, it initially came out of the Navy.
Leapfrogging had a number of advantages. It would allow the United States forces to reach Japan more quickly and not expend the time, manpower, and supplies to capture every Japanese-held island on the way. It would give the Allies the advantage of surprise and keep the Japanese off balance. The overall leapfrogging strategy would involve two prongs. A force led by Admiral Chester Nimitz, with a smaller land force and larger fleet, would advance north towards the island and capture the Gilbert and Marshall Islands and the Marianas, going generally in the direction of the Bonin Islands. The southern prong, led by MacArthur and with larger land forces, would take the Solomons, New Guinea, the Bismarck Archipelago, advancing toward the Philippines.
Ignoring the principle 
The principle of leapfrogging was not always followed in the Pacific. When MacArthur moved south to attack Mindanao after capturing the northern Philippines, and when he instigated the reconquest of portions of Borneo, he violated the "basic tenets" of island hopping. In the first case, this may have been motivated by MacArthur's promise to return to the people of the Philippines as soon as possible.
See also 
- Asada 2006, pp. 11, 18.
- On this occasion, Japan sent the cruiser Naniwa to Honolulu, Hawaii; the Naniwa arrived at Hawaii on February 23, 1893. See: William L. Neumann, "The First Abrasions" in: Ellis S. Krauss and Benjamin Nyblade, ed.s, Japan and North America: First contacts to the Pacific War, Volume 1, (London, England: RouteledgeCurzon, 2004), page 114.
- Asada 2006, p. 10.
- "Japan's Points: Hawaiian government to be asked questions." (May 4, 1897) Hawaiian Gazette, vol. 32, no 36, page 1.
- "No Arbitration: Minister Shimamura on indemnity claim." (May 7, 1897), Hawaiian Gazette, vol. 32, no. 37, page 1.
- Asada 2006, pp. 10, 11, 18, 20.
- Asada 2006, pp. 12–13, 22.
- Plan 712, Advanced Base Operations in Micronesia, iBiblio.
- Honan, William H (Dec 1970). "Japan Strikes: 1941". American Heritage 22 (1): 12–15, 91–95.
- "War in Aleutians". Life. 1942-06-29. p. 32. Retrieved November 17, 2011.
- Roehrs & Renzi 2004, p. 122.
- Roehrs & Renzi 2004, p. 119.
- Collier 1967, p. 480.
- Roehrs & Renzi 2004, p. 151.
- Asada, Sadao (2006), From Mahan to Pearl Harbor: The Imperial Japanese Navy and the United States, Annapolis, Maryland, USA: Naval Institute Press.
- Collier, Basil (1967), The Second World War: a Military History, New York: William Morrow & Co.
- Roehrs, Mark D; Renzi, William A (2004), World War II in the Pacific (2nd ed.), London: ME Sharpe.