World War II in the Pacific
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- Olympic . November 1, 1945. Invasion of Southern Kyushu to provide a large base for naval and air forces within range of Tokyo.
- Coronet . March 1, 1946. Invasion of Central Honshu and Tokyo.
Background.
Japan had captured most of the Pacific in their first six months of conquest. Stopped at Coral Sea, bloodied at Midway, reversed at Guadalcanal, the Japanese had fought fiercely to maintain her empire as American arms destroyed her naval, air, and merchant fleets. There were two drives, one by MacArthur, Army, from New Guinea to the north to return to the Philippines. A second by Nimitz, Navy, from Hawaii west across the central Pacific to obtain air bases in the Marianas within B-29 range of Tokyo.
By the beginning of 1945, Japan had been defeated but would not surrender. The steps of the end game entailed:
- 24Nov44. Start B-29 raids on Japan from Marianas.
- Feb 19-Mar 16. Iwo Jima to provide a fighter escort base within range of Japan and an emergency landing field for damaged B-29s.
- Mar 6-end. Incineration of industrial capacity.
- Apr 1-Jun 21. Okinawa to provide a base for attack planes to cover the the capture of Kyushu.
- May 8 . Germany surrenders, the Pacific, 1-year clock starts.
- May 13. Isolation of Kyushu from reinforcement starts with aerial mining.
- July 13. Isolation of Japan from their bases in China and Korea by mining.
- July 19-28. Destruction of the remaining fleet in home waters.
- Aug 06/09. Unexpected use of secret weapon, an atomic bomb.
- Aug 14 . Surrender of Japan.
OLYMPIC
Olympic entailed landing three corps on southern Kyushu, the most southern of the four Japanese home islands. The center portion of Kyushu is almost impassible mountains which would be difficult to transit and was to be used to isolate southern Kyushu from counterattack by Japanese troops fromnorthern Kyushu (Nagasaki). The landings were to be by troops already in the Pacific covered by 34 carriers and by land based aircraft from Okinawa. B-29s would interdict reinforcements. Southern Kyushu had a large bay, harbors, and many airfields. The intent was to base naval support craft and to establish 40 air groups, many redeployed from Europe. From southern Kyushu, fighter air cover could open the Inland Sea to the US Navy and interdict transportation as far as north as Osaka; fighter bombers could close shipping from Korea and China; medium bombers and could destroy transportation, material, and installations around Tokyo and support the invasion troops; large bombers (B-17 and B-24) could range over all of Japan. Meanwhile, B-29s from the Marianas could continue to wipe out industrial centers.
There were two naval groups.
The Strike Force, 3rd Fleet, had 21 carriers and 10 fast battleships to range up and down the length of Japan to suppress Japanese forces with priority to destroy aircraft and transportation.
The Assault Force, 5th Fleet, had 26 carriers, plus 8 detached from Strike force for the invasion period, 13 slow battleships, 20 cruisers, 139 DD, 167 DE, and support ships for a total of 800 warships. Troops and their equipment were to come from the Philippines and Marianas in 1,500 transports. All troops were from the Pacific theater ; none redeployed from Europe. The plan called for a diversionary display by the floating reserve on Shikoku, the smallest of Japan's four islands, before their landing on Kyushu.
The Japanese Defenses
At the time of the start to planning of Olympic, there were 1-1/2 divisions based on southern Kyushu; these with various service bases there amounted to about 45,000 men. Planners expected an additional three divisions to be moved into the area by the time of the landings. The Japanese were able to predict the landing using the same logic as US planners and moved 9 more divisions into the area for 216,000 men by the time of the surrender. Each side of the central bay had an army, each was divided into two functions - a static defense force on the beaches to fight to the death while allowing reinforcements to arrive, and the mobile reserve to push the American back into the sea. The three logical landing beaches were defended from the shore to the nearby mountains with new troops. The reserves located in the mountains were experienced troops from Manchuria with light tanks. Supporting the troops were the remnants of the navy and air force, lightly armed volunteers, and an array of "special" weapons.
The air forces contained 5,600 conventional combat planes and a similar number of older planes and trainers as Kamikazes. The combat planes were withdrawn from Kyushu and the Kamikazes moved in. Japan was so short on aircraft and fuel that B-29s and carrier task forces were not routinely attacked so as to conserve combat aircraft for the final battle. Equal numbers were assigned to Kyushu and Tokyo areas. As the date for the first battle approached, more were moved to Kyushu with the Tokyo forces to be replaced with promised new production.
Kamikaze tactics were initiated in the Philippines and became a doctrine that inflicted terrible damage to warships off Okinawa. The invasion of Kyushu would see the distance reduced, flying over familiar land instead of over-water navigation, and with targeting changed from warships to troop ships, the plan was to inflict intolerable damage to the invasion force before it hit the beach.
The following "special weapons" were established on southern Kyushu.
- Kamikazes -- 2,100 army planes and 2,700 navy planes.
- Baku - suicide missile carried by a bomber.
- Mini-subs, each with 2 torpedoes, 500 were building.
- Fleet submarines -- arm the 57 remaining that had been dedicated to resupply of outposts.
- Kaiten - suicide torpedoes with a 20 mile range.
- Shinyo - suicide motorboats. The army had 1-man, 17 foot motorboats. The navy had 2-man, 22 foot boats.
- The largest surviving warships were destroyers that were prepared for suicidal attack on the invasion convoys.
- On the land, human mines in which soldiers had explosives strapped to their bodies and were to crawl under a tank. Other explosives were packed with a suction cup to be attached to the side of a tank. And shaped charges on a long pole were to be detonated on the side of a tank.
- Paratroopers were to attack Okinawa to disrupt flight operations during the invasion period.
Alternatives to Olympic.
Rather than invade Japan, the country could have been blockaded with a ring around the Yellow Sea from Shanghai to Korea. This was not assured to cause the surrender of Japan. The direction of the war would have been towards reinforcing China and supplying the Soviet Union for their movement of troops into Manchuria, Korea, and mainland China.
A plan resurrected after the enemy buildup on Kyushu exceeded all expectations, was the occupation of the less well defended northern island of Hokkaido and northern part of Honshu. This would have been of equal distance from Tokyo, but further from American army, naval, and air force centers. Shipping was already a problem with large numbers unreleased from the Atlantic needed to supply Europe and return troops to the US, to redeploy air and service forces from Europe to the Pacific, to supply the Pacific buildup and to move several corps to the invasion sites. Every tanker in the US fleet was required to provide the millions of gallons of fuel required by the ships involved in the Kyushu operation. More fuel and shipping would be required to move 1,100 miles further away to the north.
Casualties.
There are two sets of potential casualty figures; a low number used to gain approval to proceed with the operation; and a high number used to plan reserve forces, medical needs and, as it turned out, to claim as lives saved by use of the atom bomb. These figures changed over time, starting low and going higher as the enemy build up on Kyushu was discovered.
The low casualty figures were based on the landings at Okinawa, Lingayen Gulf, and Normandy. Okinawa and Lingayen Gulf were undefended on the beaches, the fighting took place in the mountains where each Japanese caused one US wounded and 20% caused US deaths. Normandy had the same three-beach landing pattern, but two beaches were relatively easy, only the landing at "bloody" Omaha was vigorously defended. All three Kyushu beaches were defended in depth, to the death. It would have been more realistic to triple the rate of Omaha beach rather than take the average of the three Normandy beaches as the planners did. There was also a difference in scale. Normandy landed 5 divisions plus 3 airborne divisions. Olympic was to land 14 divisions. Coronet was to have 23 divisions.
The defended beach at Tarawa was a shock to Marine landing with unexpected losses. The US invasion tactic was then changed from surprise to heavy bombardment. The Japanese had changed their defensive positions in the later Pacific actions from defense of the beach to the mountains. Kyushu was to have both forms of defense: well prepared installations near the beaches and well prepared caves in the mountains.
The US planners expected that radar would detect Kamikazes coming through the mountains, carrier fighters would be vectored to intercept them, and proximity-fused ship's anti-aircraft fire would take out any that got through. However, 250 highly maneuverable warships were hit a few months earlier at Okinawa with these same defenses, in open water; the prospects of loaded troop ships taking casualties was high and each hit could take half-a-thousand lives. Whereas two Messerschmitts were able to attack troops on the Normandy beaches, 5,000 Kamikaze were aimed at the approaching troop ships while still at sea. It would be reasonable to increase the hit rate of Kamikazes from nearby bases, yet the planners reduced it. Japanese planners expected almost 500 ships would be sunk during the landing. US planners expected 15-20% losses, they had no experience with such attacks on merchant ships at sea.
Good weather was required for close air support on the cloudy islands of Japan. A typhoon had once saved Kyushu from invasion by Mongols in 1281. A storm that forced the carriers to withdraw or even to cause the 2,000 planes to remain on deck, would take away an important part of the invasion support. A storm would also hamper getting supplies over the beaches to the armies.
The original plan was for 9 divisions to attack 3 divisions of defenders. As enemy reinforcements were observed, the size of the invasion force was increased. The final plan had 18 U.S. divisions attacking 11 IJA divisions in defensive positions. Most sources give the advantage to defenders by 3:1, that is, attackers must outnumber defenders by three to be sure of victory.
Casualty figures were a guess that changed with time. There are sufficient numbers available to support any post-war position that any author chooses to take. Low numbers are quoted as reasons to do the invasion, 125,000 for Olympic and to end the war. High numbers, one million US casualties for Downfall, are quoted to justify the A-bomb and end the war. Typically, 25% of casualties are deaths. On average, 5 Japanese soldiers died for each American death.
Japanese casualties were not subjected to planning. If all troops resisted to the death, then the typical survival rate would have only included injured and unconscious soldiers. 216,627 troops were surrendered on Kyushu alone -- more than were expected -- and this was two months before the planned invasion. Civilian casualties are a real unknown. 97,000 were killed in the bombing of Tokyo on March 9; the numbers from land warfare would also be high. Consider ratios of any proportion you desire. Civilian losses in some European cities were considerable; certainly Japanese casualties would be in the multiple millions.
CORONET
Coronet was the attack across the Kanto plain to capture Tokyo. The broad plan was still going through refinement.
Naval bombardment by guns and air would begin at Y-15. There would be two simultaneous assaults on Y day.
First Army was to land on the southern half of Kujukuri Beach with 4 divisions to secure a beachhead. On Y+5, with two more divisions landed, they would move across the peninsula to clear the east side of Tokyo Bay and move north to take the port city of Choshi. Service troops would built land based air bases under the cover of carrier aircraft. Thirty air groups were expected to be in place by Y+30.
Simultaneously, Eighth Army would land at Sagami Bay with four divisions to establish a beachhead, secure the Miura Peninsula and Yokosuka naval base. At Y+10, two armored divisions would land and move straight north to establish a blocking position north of Tokyo. Other elements were tasked to take the port cities of Yokohama and Kawasaki to provide supply points for the troops.
First Army was to attack across the Kanto plain to Tokyo about Y+30 with Eighth Army tanks ready to provide assistance. Tokyo had already been extensively destroyed by bombing.
Coronet was a larger operation than Olympic, but the landing on Kyushu, the southern island, was expected to be the more expensive because all of the homeland defensives would have been expended there and the promised replacements for Coronet would have been made industrially impossible. Air Force would be flying from a hundred airfields on Kyushu. A 100 carriers could have been available including new construction and those coming from the Atlantic.
The Joint Chiefs expected the Japanese to surrender after exhausting themselves in Operation Olympic. Thus Coronet would not be required.
However, if necessary, follow up operations after Tokyo would have been initiated in the south, central, and north of Japan with US troops from Europe who had taken leave in the US -- only Air Force, air field construction, and service units had gone from Europe directly to the Pacific. And troops from Allied countries would be available.
- South. The northern, more industrialized half of Kyushu would have been taken.
- Central. The next largest industrial cities would have been taken with landings to take the peninsulas of Ise Bay, take Nagoya and then march overland to Osaka, Kyoto and Kobe.
- North. A landing at Sapporo on Hokkaido followed by taking the anchorage at Mautsu.
Hindsight . Capture of the Marianas as B-29 bases turned out to be the key to the end-game. The campaign in the Philippines and Peleliu were not necessary except to force the final battle with the Japanese fleet and as a place to absorb Japanese army troops and air forces for destruction. However, the alternative considered at the time, the invasion of Formosa to open access to China and as an alternate base for B-29s would have been equally unnecessary. These resources could have been applied sooner to capture and build air bases at Saipan, Iwo Jima and Okinawa, which were instrumental in the rapid end of the war. But the secret weapon was a secret and it might not have worked or been ready in time.
Troops . All were US from Pacific area.
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Coronet : Autumn of 1945 6th Army
Sep 1 . Honshu, Kyushu, Strategic Air Force (B-29 Okinawa) ; continue strategic targets.
Sep 1 . Shimonoseki Straight / ports, Strategic Air Force ; continue isolation mining.
Sep 18. Hong Kong , British strikes.
Sep 28. Canton , British strkes.
Oct 1 . Ningpo , Chusan, China , Strategic Air Force ; isolation bombing.
Oct 18. Honshu, Inland Sea , 3rd Fleet : TF-38 (US) , TF-37 (UK) ; strategic support
Oct 21. Kyushu , Strategic Air Force ; N-S isolation and anti-buildup
Oct 24. Kyushu , 5th Fleet ; preliminary bombardment, mine clearing, interdict highways.
Oct 27. Outer Islands , 40th Inf Div
Oct 28. Tanega Shima , 158th Reg Combat Team
Oct 30. Shikoku , feint by 9th Corp : 77th , 81st , 98th Infantry Divisions
Nov 1 . West , 5th Amphibious Corp : 2nd , 3rd , 5th Marine Divisions
Nov 1 . South , 11th Corp : 1st Cav , 43rd Inf , Americal Divisions.
Nov 1 . East , 1st Corp : 25th , 33rd , 41st Infantry divisions
Nov 22. Where needed : 11th Airborn Division.
Nov 23. As needed or SW : 9th Corp : 77th , 81st , 98th Infantry Divisions
Dec - . Build air fields
Jan - . Attack all military and industrial areas of Japan.
Further Information.
See the short version of this page.
"The Invasion of Japan:" by John Ray Skates, U of So Carolina Press, 1994. Well researched but seems to draw the conclusion that invasion was cheaper than the bomb. [to whom?]
"Code-Name Downfall: The Secret Plan to Invade Japan and why Truman Dropped the Bomb" by Thomas B. Allen and Norman Polmar, Simon & Schuster, 1995
There have been many magazine articles over the years. There are many differences in various reports because Operation Downfall was a plan that evolved over various times. Check the web, there are lots of sites shown by a search.
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About this page: Downfall - Description of the plans for the invasion of the home islands of Japan in the Fall and Spring of 1945-46.