why not bypass small islands
A lot of Japanese-occupied islands were indeed bypassed. The fleet anchorage at Truk, for instance; it played host to a significant portion of the Japanese Kaigun and saw only air raids. By 1945 Truk was so irrelevant that the British Pacific Fleet used it as a trial run for some of their carriers on 1945 June 14-15. Rabaul, one of the strongest Japanese fortifications, also saw no invasion.
I commend to your attention the following previous threads:
And we still welcome further contributions from other users, so if you'd like to put in your own oar about island-hopping, please don't let this post stop you!
It's a little difficult to understand what your precise question is, but you might find the answer you're looking for in these older threads:
If your question boils down to something like "Why not just invade the Japanese home islands directly in 1942/43/44/45 instead of the Solomons/New Guinea/Gilberts/Marshalls/Marianas?" then there are two broad issues. The first is how to deliver enough men and material to Japan while keeping that force protected and supplied. The second issue would be the warmaking capacity of Japan. I'll elaborate on both in turn here.
It would have been difficult and dangerous to transport an invasion force directly to the Japanese home islands. Early in the war, it likely would not have been possible. There would have been numerous problems, any one of which might have been enough to lead to a defeat.
For starters, the Allies would have lacked the airpower necessary to execute and sustain a large-scale invasion of the Japanese home islands. An amphibious invasion of the Japanese home islands before late 1944 (after the Marianas campaign in summer 1944) would have been wholly reliant on carrier-based aircraft for air support. Even then, the Marianas were too far away for tactical land-based air support; only the B-29s had the range to reach the Japanese home islands. That means that Japan would have been able to use its land-based aircraft to strike at any invasion force and whittle it down as it sat in the ocean, thousands of miles away from repair facilities. The Japanese fleet also would have been able to use its aircraft carriers and submarines as a mobile threat to the Americans. Attacks would have come from all sides. Once the American fleet had taken enough damage and the carrier airpower was neutralized, the Japanese battle line might have closed the distance for the final blow (or aircraft might have just kept up the attack). After 1942, the Americans never carried out an amphibious invasion in the Pacific without air superiority or air dominance. Even in 1942 in the Solomons, the Americans established local air superiority by seizing Henderson Field at Guadalcanal, thus requiring Japanese aircraft to fly hundreds of miles for only a few minutes of combat before they had to return home.
This was one of the primary differences between the invasion of Normandy -- which included zero aircraft carriers -- and most Pacific invasions, which usually relied on aircraft carriers to provide cover for the invasion fleet and tactical air support for the invasion force. The Normandy invasion fleet is sometimes billed as the largest fleet in history. Purely by the number of hulls involved, that is true. However, the invasion fleet at Okinawa included more than a dozen large aircraft carriers and more than two dozen smaller aircraft carriers to provide the necessary air cover. The invasion fleet still took heavy casualties from Japanese air attacks (mostly kamikazes). There were also more battleships to provide naval fire support at Okinawa than Normandy; the allies didn't have the advantage of using airfields to provide air support for the invasion like they did with Great Britain. The Allies probably would not have had an adequate carrier force to do this until 1945, and even in the plans for the fall 1945 invasion of Kyushu, it was to be supported by land-based aircraft from Okinawa. The general plan for invasions in 1944 and 1945 included a period where the carrier aircraft would destroy enemy airfields and aircraft before the invasion if possible, so that during the invasion the aircraft could bomb defensive positions and protect the fleet from enemy ships or aircraft. This was obviously easier when attacking smaller islands (such as Peleliu) than larger islands that could have dozens of airstrips and concealed aircraft in jungles (like the Philippines). Attacking the Japanese home islands would have been far more difficult than the Allies were prepared for.
The Japanese Navy was a real threat to the U.S. Navy until after the Marianas campaign and the Battle of the Philippine Sea in summer of 1944, when there was still a sizeable force of Japanese aircraft carriers, aircraft, and pilots. Any invasion fleet would have had to defeat the Japanese Navy before it could think about safely landing troops, and that would have been difficult for the Allies before 1945. As mentioned above, the fleet would have been at a tremendous disadvantage due to the presence of enemy land-based aircraft that could greatly outnumber them, and the Japanese fleet itself likely would have met the Allies on roughly equal terms. The Allies were carrying out amphibious invasions in North Africa, Sicily, Italy, and France from 1942-44, all of which demanded warships and amphibious landing craft that would have been required for a large-scale invasion of Japan. As mentioned in another thread, the invasion of the Solomons (the Guadalcanal campaign in late 1942) was nicknamed “Operation Shoestring” because of how desperately short of resources the attack was. Operations in the Gilberts in late 1943, most notably Tarawa/Betio, didn’t have enough tracked amphibious craft to drive over shallow reefs that landing boats couldn’t sail over.
The quick version of the Pacific War might leave you with the impression that the Japanese fleet was declawed after Midway. While the Japanese never had the naval assets to undertake another offensive, the American and Japanese fleets usually met on roughly equal terms for most of 1942 and early 1943. The Solomons campaign was a meat grinder that sucked in men, aircraft, and ships from both sides. But the Americans could accept equal losses if necessary with the knowledge that American industry could build new ships and aircraft much, much faster than Japan.
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