Japanese Preparations
ALTERNATIVES TO “OLYMPIC”
Report by the Joint Staff Planners
1. In Enclosure “B” (page 3) the Service Members of the Joint Intelligence Committee report a considerable strengthening of Japanese forces in southern Japan proper. Along with an increase in ground units deployed in southern Kyushu, a concentration of aircraft, including the bulk of Japanese suicide aircraft, and small suicide naval craft is reported in the area.
2. The possible effect upon OLYMPIC operations of this build-up and concentration is such that it is considered commanders in the field should review their estimates of the situation, reexamine objectives in Japan as possible alternates to OLYMPIC, and prepare plans for operations against such alternate objectives.
3. The Joint Staff Planners are preparing studies of alternate objectives in light of current intelligence estimates. These studies are to be made available to theater commanders upon completion.
APPENDIX TO ENCLOSURE “A”
DRAFT
MESSAGE TO CINCPAC AND CINCAFPAC
INFORMATION TO CG, USASTAF
1. Copies of highly secret reports by the Joint Intelligence Committee (J.I.C. 311, Defensive Preparations in Japan) have been furnished you. Report indicates strengthening of Japanese forces and defensive measures in southern Japan to an extent considerable in excess of that previously estimated as Japanese capability by OLYMPIC target date. While these measures on the part of the Japanese are not yet considered to require change to your current directive it is desired that you give continued consideration to the situation particularly as it affects the execution of OLYMPIC, make alternate plans and submit timely recommendations. Operations against extreme northern Honshu, against the Sendai area, and directly against the Kanto Plain are now under intensive study here.
GENERAL SUMMARY
2. In anticipation of Allied invasion of the Home Islands, the Japanese are making a maximum effort to strengthen their defensive capabilities in Japan Proper and to redeploy their forces in accordance with their own estimate of areas most likely to be invaded. Preparations for defense are substantiated by the following evidence: (a) ground forces are being expanded at a greatly accelerated rate; (b) a policy of rigid conservation of aircraft has been adopted, new airfields constructed, and air strength being deployed to permit the most effective utilization against an invading force; (c) stress is being laid upon the extensive defensive mining of approaches to threatened areas an the establishment of small craft suicide attack bases in these areas; (d) emphasis is being given to coordinated ground and air action to frustrate our landing operations supplemented by small craft and ingenious weapons to be employed in suicidal tactics. We believe that the Japanese have accorded priority for defensive preparations in the following order: (1) Southern Kyushu; (2) Shikoku; (3) Northern Kyushu; and (4) Kanto Plain. Apparently, the Japanese anticipate heavy destruction of their communication lines and are now attempting to concentrate the greatest part of the forces to be used for the defense of these vital areas in close proximity to the most threatened points of probable Allies assault.
PARTICULAR AREAS
3. There is every indication that the Japanese have been giving the highest priority to the defense of Kyushu and particularly Southern Kyushu.
Since early 1945, ground forces deployed in Kyushu have been increased from 1 active and 2 depot division (totaling with army troops some 150,000 men), to a present strength of 11 active and 2 depot divisions (totaling with army troops about 545,000 men). During recent months, 2 divisions were brought in from Manchuria, 1 from Hokkaido, 2 from Honshu, and 1 from an unknown location in Japan, while 4 active divisions were formed locally from depot divisions. In addition, 1 independent mixed brigade and 3 tank brigades have been recently identified in Kyushu and one amphibious brigade has been transferred from the Kuriles to Southern Kyushu. In early 1945, about 75% of all ground forces on this island were located in Northern Kyushu, whereas now 7 of the 11 active divisions and about 60% of the total strength are deployed in Southern Kyushu.
Recent trends in the deployment of elementary biplane trainer type aircraft for suicide operations likewise point to the defensive priority which is being given to the Kyushu area by the Japanese. A total of 50 special bases for suicide aircraft are thus far known to have been designated in Kyushu, Honshu, and Shikoku west of 133° longitude. The Japanese naval air forces alone have deployed in this area 1,200 biplane trainers designated for suicide missions at the time of invasion. In comparison, only 32 such fields, 400 navy biplane trainers and 400 navy monoplane trainers have been so designated in the area between 133° and 138°, while only 22 suicide bases and 600 navy biplane trainers are thus far located in the remainder of Honshu, which includes the Tokyo Plain.
Due to their greater range and mobility, the deployment of combat aircraft is less indicative of defensive priorities than in the case of elementary biplane trainers, but it is significant that the major portion of combat aircraft in Japan Proper is deployed or scheduled to be deployed at bases west of 138° E., thus placing tactical emphasis on the Kyushu-Shikoku area.
The trend of aircraft fuel accumulations seems to be generally in line with the present and prospective deployment of combat and trainer aircraft.
The immobilization and destruction of practically all large combatant units of the Japanese Navy has permitted release of personnel, possibly exceeding 100,000 men for other assignments. There is evidence that many of these men from the Kure and Sasebo area will be utilized for intensifying defensive preparations in Kyushu and nearby areas. Special naval landing forces are undergoing combat training to support army ground forces in defense against Allied landings. In addition, numerous suicide attack units are being formed and trained for employment in one-man torpedoes, midget submarines, small suicide boats, and underwater swimming teams. There is also a strong possibility that a majority of Japan’s remaining destroyers will be employed in suicidal missions against surface forces.
Many defensive mine fields have been laid and there is considerable evidence that new types of mines which might be effective against Allied landing craft are being set out in waters of less than 15 fathoms along prospective landing beaches. A very high priority has been given to the Kyushu area, the most extensive mining taking place south of 33° latitude. We believe that these mining and other naval defensive preparations in Southern Kyushu will be completed by September. Already, it is believed, an extensive min field has been completed along the east coast of Kyushu from Tomitaka to Miyazaki. In addition, other mine fields have been laid along the approaches to Ariaki Wan, Kagoshima Wan, and Tachibana Wan.
4. Shikoku. Defensive preparations in Shikoku would seem to indicate that the Japanese estimate the probability of Allied landings on this island as being second only to invasion of Kyushu or Quelpart Island.
In early 1945 no active divisions were known to be located in Shikoku, the available ground troops at that time consisting of 1 depot division and miscellaneous units totally some 50,000 men. In recent months, however, 1 division has been brought in from Manchuria and 2 from Central and Western Honshu, and 1 active division has been created locally. Present strength, including 4 active divisions, 1 active division has been created locally. Present strength, including 4 active divisions, 1 depot division, and miscellaneous units, totals about 150,000 men. Recent incorporation of the ground forces in Shikoku into an area Army headquarters indicates the importance which the Japanese attach to the defense of this island.
Present deployment of the Japanese air forces, as described in paragraph 3 above, afford the Japanese equal capability for the air defense of Shikoku as compared with Kyushu. From their present bases all of the aircraft based in Japan Proper west of 138° would be within range of our assault forces attacking Shikoku.
From the naval standpoint, the Japanese are believed to have established some bases for suicide attack units in the Kochi (Lat. 33° 33’ N, Long. 133° 33’ E) area. Mines have been laid in the waters along the south coast of Shikoku in the vicinity of Kiochi and some of these mines may be of the shallow water type for defense against Allied landing craft approaching the beaches. Defensive minefields have also been laid in Bungo Suido between Kyushu and Shikoku.
5. Kanto Plain. While there is considerable evidence that the Japanese expect us to make initial landings in Kyushu, Shikoku or some other area of Japan Proper prior to an assault on the Kanto Plain, they do not exclude the possibility of amphibious operations against this latter area from or present positions.
In early 1945 there were only 4 active and 3 depot divisions, plus army troops totaling about 300,000 men, located in the vicinity of the Kanto Plain. Progressive reinforcement of the area has been carried out since March, 1 armored division having been brought in from Manchuria, 2 active divisions from other parts of Japan Proper, wile at the same time 2 division were activated locally from depot divisions. Present strength in this area is estimated to be 9 active divisions, 3 depot divisions and army troops, totaling about 560,000 men. No information is available regarding any unusual recent activity in the strengthening of coastal defenses, although there is every reason to believe that here, as well as in other threatened areas of Japan Proper, fixed defenses are being constantly developed and improved.
From the air standpoint, while as stated in paragraph 3 above, there are at present only 22 special suicide bases known to be located east of 138°, it must be born in mind that all trainer type aircraft as well as combat types based in Central Honshu would be within easy range of the Kanto Plain area. At least one large, specially concealed air base is being hastily constructed in the vicinity of Koriyama through which a large number of trainer-type aircraft could be staged, or at which 100 suicide trainers could be based. Thus, while tactical emphasis in aircraft deployment points towards the Kyushu-Shikoku area, it is obvious that large numbers of trainer-type as well as combat aircraft will be in a position to operate effectively in the defense of the Kanto Plain.