question: During WW2, why did the US engage the Japanese on the Pacific islands instead of blockading them and going straight for the main Japanese island?
comment: You need a base to refuel and launch aircraft, especially for a Long campaign. The aircraft carriers of ww2 simply did not have the aircraft capacity or refueling capability to project air power all the way to japan. They had to take some of the islands to “leapfrog” their way towards japan.
passages: American plans changed after this attack. Even after major Japanese defeats like Midway, once the effectiveness of aircraft carriers was known, the Americans favored a methodical "island-hopping" advance, never going far beyond land-based air cover. Meanwhile, a blockade was imposed from the very beginning of the war, with the first American submarine, USS "Gudgeon", arriving off Japan on about 31 December 1941.
   Both sides were overextended in the exhaustive sea, air and land battles for Guadalcanal. The Japanese were better at night combat (because the American destroyers had only trained for attacks on battleships). However, the Japanese could not feed its soldiers so the Americans eventually won because of superior logistics. The Navy built up its forces in 1942-43, and developed a strategy of "island-hopping, that is to skip over most of the heavily defended Japanese islands and instead go further on and select islands to seize for forward air bases.
   After the attack on Pearl Harbor, there was considerably more JN-25 traffic as the Japanese Navy operational tempo increased and geographically expanded, which helped progress against it. Hong Kong's contribution stopped until the crypto station there could be relocated (to Ceylon and eventually Kenya), but and the Dutch at Batavia, in conjunction with and OP-20-G made steady progress. in particular made significant contributions. Its people, including its commander, Joseph Rochefort, thought a forthcoming Japanese attack early in 1942 was intended for the central Pacific, while opinion at OP-20-G, backed by , favored the North Pacific, perhaps in the Aleutians.
   As the island hopping toward Japan continued, AFRS became more and more popular among the troops. The term "island hopping" is often used to describe the way in which Allied forces advanced toward Japan, mainly because many Japanese-held islands were literally bypassed, or hopped over. Some were neither captured nor occupied by Allied forces until after the official Japanese surrender.
   Section::::Background.:Japanese defences.
   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, Major 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 reporter on naval affairs, Hector Charles Bywater, publicized the prospect of a Japanese-American war in his books "Seapower in the Pacific" (1921) 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.
   Supply runs were a lesser drain on Allied resources. The need to supply MacArthur's forces trapped in the Philippines led to diversion of boats to "guerrilla submarine" missions. As well, basing in Australia placed boats under Japanese aerial threat while "en route" to patrol areas, inhibiting effectiveness, and Nimitz relied on submarines for close surveillance of enemy bases. A small number of oversized submarines handled much of the resupply, submarines that were less agile than their sisters attacking escorted convoys.
   BULLET::::- In the same period, Japanese forces took the Rabaul bases in the Admiralty Islands in the New Guinea area and defeated the German naval force of Admiral Von Spee, who ordered a withdrawal to the East Pacific area.
  
  BULLET::::- At the same time, Japanese landed from Formosa and took for a while the Chinese province of Fujian, also promoting some revolts against the Chinese central government.
   For the next five months, "Neosho" sailed with the aircraft carriers or independently, since escort ships—now few and far between—could not always be spared to guard even so precious a ship and cargo. Late in April, as the Japanese threatened a southward move against Australia and New Zealand by attempting to advance their bases in the Southwest Pacific, "Neosho" joined Task Force 17 (TF 17). At all costs, the sea lanes to the dominions had to be kept open, and they had to be protected against attack and possible invasion.
   In the years immediately preceding the Second World War, the U.S. had no standing clandestine HUMINT or covert action organizations. There were certainly examples of both, such as Marine Major Earl Ellis' series of visits, in the 1920s, to Japanese islands in the Pacific. Ellis, who died under mysterious circumstances while on duty, created the basic plan for U.S. "island hopping" operations in the Pacific Theater of the Second World War.
   US Navy strategic stations targeted against Japanese sources at the outbreak of the war, included Station HYPO in Hawaii, Station CAST in the Philippines, station BAKER on Guam, and other locations including Puget Sound, and Bainbridge Island. US COMINT recognized the growing threat before the Pearl Harbor attack, but a series of errors, as well as priorities that were incorrect in hindsight, prevented any operational preparation against the attack. Nevertheless, that attack gave much higher priority to COMINT, both in Washington, D.C. and at the Pacific Fleet Headquarters in Honolulu. Organizational tuning corrected many prewar competitions between the Army and Navy.
   BULLET::::- 1942-01-25 Thailand declares war on the Allies
  
  BULLET::::- 1942-01-30 – 1942-02-03 Battle of Ambon
  
  BULLET::::- 1942-01-30 – 1942-02-15 Battle of Singapore
  
  BULLET::::- 1942-02-04 Battle of Makassar Strait
  
  BULLET::::- 1942-02-14 – 1942-02-15 Battle of Palembang
  
  BULLET::::- 1942-02-19 Air raids on Darwin, Australia
  
  BULLET::::- 1942-02-19 – 1942-02-20 Battle of Badung Strait
  
  BULLET::::- 1942-02-19 – 1943-02-10 Battle of Timor
  
  BULLET::::- 1942-02-27 – 1942-03-01 Battle of the Java Sea
  
  BULLET::::- 1942-03-01 Battle of Sunda Strait
  
  BULLET::::- 1942-03-01 – 1942-03-09 Battle of Java
  
  BULLET::::- 1942-03-31 Battle of Christmas Island
  
  BULLET::::- 1942-03-31 – 1942-04-10 Indian Ocean raid
  
  BULLET::::- 1942-04-09 Bataan Death March begins
   BULLET::::- Operation Ta 1943 - Japanese attack on Bougainville in the Solomon Islands
  
  BULLET::::- Operation A-Go 1944 - Japanese attack of the American fleet off Saipan, Mariana Islands
  
  BULLET::::- Operation Ban-Go 1944 - Japanese defensive operation at the Irrawaddy River in Burma
  
  BULLET::::- Operation Dan-Go 1944 - Japanese operation after the failed operation U-Go offensive in India
  
  BULLET::::- Operation FS 1944 - Japanese cancelled plan to isolate Australia by capturing New Caledonia, Samoa, and Fiji
  
  BULLET::::- Operation Fu-Go 1944 - Japanese aerial bombing of the United States via balloons
   The sneak attack was partly motivated by a desire to cripple the United States Pacific Fleet in order to allow Japan to advance with reduced opposition from the U.S. when it, in order to secure Japanese oil supplies, fought the British Empire and the Dutch Empire for control over the rich East Indian (Dutch East Indies, Malay Peninsula) oil-fields. In 1940, American policies and tension toward Japanese military actions and Japanese expansionism in the Far East increased. For example, in May 1940, the base of the U.S. Pacific Fleet that was stationed on the West Coast of the United States was forwarded to an "advanced" position at Pearl Harbor in Honolulu, Hawaii.
   After 7 December 1941, there was considerably more JN-25 traffic as the Japanese Navy operational tempo increased and geographically expanded, and progress against it went better. The FECB contribution stopped until the crypto station could be relocated from Singapore to Ceylon, but CAST, HYPO, and the Dutch at Batavia, in conjunction with OP-20-G, made steady progress.
  
  Section::::CAST during and after the invasion of the Philippines.
   Many Americans feared that the Japanese would use the islands as bases to strike within range along the rest of the United States West Coast. Although the U.S. West Coast was subject to attack several times in the past six months (including unrestricted submarine warfare in coastal waters and the bombardment of Ellwood in Santa Barbara, California), the Aleutians Islands Campaign of June 1942 was the first major operation by a foreign enemy in the American Theater of the war. Lieutenant Paul Bishop of the 28th Bombardment Group once recalled that:
   Section::::Background.:Defenses.
  
  Section::::Background.:Defenses.:USAFFE.
  
  From mid-1941, following increased tension between Japan and several other powers, including the United States, Britain and the Netherlands, many countries in South East Asia and the Pacific began to prepare for the possibility of war.
   BULLET::::- 1945-02-16 – 1945-03-26 Battle of Iwo Jima
  
  BULLET::::- 1945-04-01 – 1945-06-21 Battle of Okinawa
  
  BULLET::::- 1945-04-07 Operation Ten-Go
  
  Borneo campaign
  
  BULLET::::- 1945-05-01 – 1945-05-25 Battle of Tarakan
  
  BULLET::::- 1945-06-10 – 1945-06-15 Battle of Brunei
  
  BULLET::::- 1945-06-10 – 1945-06-22 Battle of Labuan
  
  BULLET::::- 1945-06-17 – 1945-08-15 Battle of North Borneo
  
  BULLET::::- 1945-07-07 – 1945-07-21 Battle of Balikpapan
  
  Japan campaign
  
  BULLET::::- 1945-07-22 Battle of Tokyo Bay
  
  BULLET::::- 1945-08-06 – 1945-08-09 Atomic bombings of Hiroshima and Nagasaki
  
  Soviet invasion of Manchuria
  
  BULLET::::- 1945-08-08 – 1945-09-02 Soviet invasion of Manchuria
  
  Section::::Command areas.
   BULLET::::- Continuing with previous Meiji period precedents, the Japanese Navy sent some submarines on missions on the American Pacific coast. One of them struck at ports in California, and another launched a small reconnaissance seaplane to bomb the Oregon area. Some heavy flying boats from the Marshall Islands area stuck Hawaii. An apparently diversionary invasion of the Aleutian islands in Alaska was launched, Fu-Go balloon bombs were sent to the Pacific coast. Some plans were made to attack the U.S. Pacific cities of Los Angeles and San Francisco, and to strike at the Panama Canal. The Japanese also conducted similar reconnaissance missions in Mexico, Central America and the South American Pacific coastal areas.
   BULLET::::- Attack on Pearl Harbor 7 December 1941
  
  BULLET::::- Battle of Wake Island 7–23 December 1941
  
  BULLET::::- Philippines Campaign (1941–42) 8 December 1941 - 8 May 1942
  
  BULLET::::- Doolittle Raid 18 April 1942
  
  BULLET::::- Battle of the Coral Sea 4–8 May 1942
  
  BULLET::::- Battle of Midway 4–7 June 1942
  
  BULLET::::- Guadalcanal Campaign 7 August 1942 to 9 February 1943
  
  BULLET::::- Gilbert and Marshall Islands campaign 1943–44
  
  BULLET::::- Makin Island raid 17–18 August 1942
  
  BULLET::::- Battle of Tarawa 20 November 1943
  
  BULLET::::- Battle of Makin 20–23 November 1943
  
  BULLET::::- Battle of Kwajalein 14 February 1944
   Section::::History.:Aleutian Campaign 1942.
   BULLET::::- Japan extended its submarine long-range reconnaissance missions to the Asian and African coasts of the Indian Ocean, to Australia, New Zealand and other islands, and to the Western and South Pacific seas.
   When the new American fast battleships began to arrive in the Pacific in the summer of 1942, they were allocated to the carrier task forces where their heavy anti-aircraft batteries could defend the vulnerable carriers, rather than being formed into separate battle squadrons. By 1943, as growing numbers of new carriers, battleships, cruisers and destroyers began to reach the Pacific, the Americans developed a fleet of fast carrier task forces that swept across the Pacific over the next two years, isolating, overwhelming and then destroying the Japanese island bases.
  
  Section::::Sources & references.
   Japan's primary goal after World War I was to expand its economic influence and control in East Asia, principally China. In that strategic aim Japan faced opposition from Britain and the Netherlands, who held colonial interests in the region, and from the United States, which sought to protect its territories in Guam and the Philippines, and maintain an economic Open Door Policy in China. In this setting, the naval planners of both Japan and the United States began working out scenarios for how a possible conflict in the Pacific would be fought and won.
   By the late 19th century, the U.S. had several interests in the western Pacific to defend; namely, access to the Chinese market, and its 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.
  