Foreign Relations of the United States, 1946, The Far East, Volume VIII

711.94/1–2247

Memorandum Entitled “Reconstruction of Japanese Plans Leading Up to the Attack on Pearl Harbor”⁴⁹

undated

The following summarization has been prepared on the basis of reliable information obtained from various sources subsequent to the attack on Pearl Harbor. Although this summary does not represent a résumé of Japanese Combined Fleet Operation Order No. 1,⁵⁰ it will be apparent that heavy reliance has nevertheless been placed upon that document, a translation of which is submitted under separate cover.⁵¹

The Japanese Naval High Command completed preparations during the summer and fall of 1941 to carry out a projected 2–phase plan of conquest and consolidation in the Asiatic-Pacific theatre. The essence of the plan lay in the element of surprise in a sudden attack which at one fell swoop would cripple the major potential opposition—the American Fleet maintained at Pearl Harbor. Following the crippling of this opposition weapon, coincident with the seizure and occupation of land masses desired for the economic, political, and military reasons that together determined the borders of the Greater East Asia Co-Prosperity Sphere, Phase 1 of the campaign would be completed, and Phase 2—the consolidation of these gains by the seizure or neutralization of outer areas, together with the continued attrition of the enemy forces and his lines of supply—could be carried out to ensure the permanence of the new Empire.

The Japanese fleet, which had been in training over a period of years for combat fleet operations, was in a state of readiness by the summer of 1941. By late August of 1941, there is evidence based on information believed to be reliable, that the Commander in Chief of the Combined Fleet⁵² ordered all fleet commanders and their key staff members to Tokyo for war games^* preparatory to a final formulation of operation plans. The final games reportedly got underway on 2 September 1941, with most of the high ranking officers participating on one of the three teams that were organized: the “N” (Nippon) Team, “A” (America) Team, and “E” (England) Team.

During the afternoon of 3 September, 50 copies of an outline of conditions under which the games were to be held were prepared and contained the heart of Operation Order No. 1. These plans must have been under development for several months, as extensive preliminary planning was indicated.

On 5 September, it is known that Pearl Harbor plans were under discussion and the Japs apparently expected to catch all major U.S. Fleet units in the Pacific in Pearl Harbor, as well as units which they believed were recently transferred from the Atlantic. “N” Team expected to lose one-third of the units participating in the attack on Hawaii and one Akagi-class aircraft carrier and one Soryu-class aircraft carrier were estimated as sunk.

On 6 and 7 September, “N” Team debated the best means of assaulting Pearl Harbor. Captain Kurojima (Deputy Chief of Staff) and Rear Admiral Ito (Chief of Staff)⁵³ differed as to the practicability of conducting an amphibious assault on Hawaii. Ito was in favor of an early landing but Kurojima won the discussion by pointing out insuperable logistic problems.

These early sessions apparently were confined primarily to two general problems: first, the details for a surprise raid on Pearl Harbor; and second, a schedule for occupying Malaya, Burma, N.E.I., the Philippines, the Solomons, and Central Pacific Islands, including Hawaii. The conferences and games were ended about 13 September. At the end of September, the Main Body of the Jap Fleet moved to Saeki and four revisions of Combined Fleet Operation Order No. 1 were made while Nagato (Flagship) was at Saeki, although no major changes are indicated.

The actual operation plan itself—Combined Fleet Top Secret Operation Order No. 1—has since been recovered and is reproduced in its entirety under separate cover. The objectives of the campaign strategy, the outline and sequence of operations planned, and the organization of the naval forces allotted to each phase of the operations were set forth in the Operation Order, and are analyzed below.

Objectives:

The general aims of the entire campaign were predicated on the desires for military conquest and security, and enhancement of the Empire by the occupation of areas rich in natural resources:

• “1. In the east, the American Fleet will be destroyed and American lines of operation and supply lines to the Orient will be cut.
• “2. In the west, British Malaya will be occupied and British lines of operation and supply lines to the Orient, as well as the Burma Road, will be cut.
• “3. Enemy forces in the Orient will be destroyed, bases of operations will be seized, and areas with natural resources will be occupied.
• “4. Strategic areas will be seized and developed; defenses will be strengthened in order to establish a durable basis for operations.
• “5. Enemy forces will be intercepted and annihilated.
• “6. Victories will be exploited to break the enemy’s will to fight.”

Outline of Operations:

For the accomplishment of the objectives stated above, it was contemplated that operations would be carried out in two phases—the First Phase, comprising conquest of the American Fleet and occupation of areas desired, in large part, for economic reasons (the Philippines, British Malaya, Netherlands East Indies); the Second Phase, comprising consolidation of these gains by mopping-up operations, establishment of advance bases for defense of the occupied territory, and the continued attrition of enemy forces and lines of communication. The conquest or neutralization of areas deemed of strategic importance primarily from the point of view of military security was to occur during the Second Phase, no definite plan being provided initially for the chronology of those operations. Apparently both the precise timing of that portion of the campaign and the determination of which of these strategic areas (listed as Eastern New Guinea, New Britain, Fiji, Samoa; Aleutians and Midway; Andaman Islands, strategic points in the Australia Area) would be seized and which merely neutralized, were problems to be worked out in detail following the completion of the occupation of the areas desired for their economic value as integral portions of the new Co-Prosperity Spheres, and to be solved “as quickly as operational conditions permit.”

A. Occupation: “First Phase Operations”

Basically, at this initial stage of the war, the Imperial Navy had four missions to fulfill:

(i) The destruction of the American Fleet in the Hawaiian area.

(ii) The maintenance and extension of control over the Central and South Pacific, to deny these waters to any force which might menace the flank of the forces driving southward.

(iii) The support of army invasion of the Philippines–N.E.I.–Southeast Asiatic areas and the destruction of Allied naval forces therein.

(iv) The protection of the North, both against thrusts by the United States from the Aleutians and also against a possible attack by the U.S.S.R.

Aside from the considerably inferior air power and the relatively few scattered surface fleet units possessed by the Allies in the Asiatic–N.E.I. area, the only obstacle of consequence was the American Fleet and air-power based at Hawaii. While Japanese land-based air and surface task groups could suffice to support the amphibious landings in the Philippines–N.E.I.–Asiatic area, a major Japanese task force, built around a carrier striking group, was essential to conduct a surprise attack on the American Fleet. Accordingly, the following general allocation of Japanese forces^* was planned for the First Phase operations.

(i) For the Pearl Harbor Attack:

The Striking Force under the Commander in Chief, 1st Air Fleet,⁵⁴ comprising 2 fast battleships, 6 first-line carriers (with a maximum of 400 planes of all types), 2 heavy cruisers, 1 light cruiser, and 16 destroyers plus,

The Advance Expeditionary Force under the Commander in Chief, 6th Fleet,⁵⁵ comprising 1 training cruiser, 2 light cruisers, 20 fleet submarines, and 5 midget submarines.

(ii) For the Invasion of Wake and Guam (and of Rabaul if conditions warranted):

The South Seas Force under the Commander in Chief, 4th Fleet,⁵⁶ comprising 4 heavy cruisers, 3 light cruisers, 1 training cruiser, 12 destroyers, 16 submarines.

(iii) For the Invasion of the Philippines–N.E.I.–Malaya:

The Southern Force under the Commander in Chief, 2nd Fleet,⁵⁷ comprising 2 battleships, 2 small aircraft carriers, 11 heavy cruisers, 7 light cruisers, 52 destroyers, 16 submarines.

(iv) For the protection of the High North:

The Northern Force, under the Commander in Chief, 5th Fleet,⁵⁸ comprising 1 heavy cruiser, 2 light cruisers, 2 destroyers.

(v) In Reserve:

The Main Body under the Commander in Chief, Combined Fleet,⁵⁹ comprising 6 battleships, 2 light cruisers, 8 destroyers.

The operations of these forces during the First Phase were to be divided into three periods:

(a) First Period Operations:

Operations from the outbreak of war until the main body of the invasion army had been landed in the Philippines. To end about X plus 20.

(b) Second Period Operations:

Operations after (a) and until the main body of the invasion army had been landed in British Malaya. To end about X plus 40.

(c) Third Period Operations:

Operations after (a) and until the completion of the occupation of the Netherlands East Indies.

(i) Central Pacific Operations:

(a) Attack on Pearl Harbor

Assumptions by the High Command:

It is clear from a study of the operation plans that the Japanese High Command made the following assumptions about the American Fleet:

(a) That the main body of the United States Pacific Fleet would be at anchor within Pearl Harbor, or at least in the Hawaiian area.

(b) That a fast carrier force could be moved from the Empire across the Pacific to the north of Midway, within striking distance of the main islands of the Hawaiian group without undue risk of detection by American defensive reconnaissance.

(c) That should assumption (a) or (b) be in error, a reserve group of heavy units could sortie from the Inland Sea to give support to the carrier striking force in a decisive engagement against the American Fleet. The other task forces of the Japanese Fleet—the Southern Area Force, Northern Area Force, South Seas Force—would also be available. Implicit in the plan is the assumption that in the event of such an engagement, the combined strength of the bulk of the Japanese major fleet units could defeat the American Fleet.

(d) That a powerful carrier air strike directed against the American forces based in Hawaii could, if tactical surprise were effected, achieve the strategic result of crippling the American Fleet, and the tactical result of destroying the American land-based air to permit the Japanese striking force to withdraw without damage. While the latter assumption does not appear explicitly in the copy of the Japanese Combined Fleet Operation Order No. 1 recovered in the Philippines, it is logically implicit in the plan, and probably was a feature of the specific Operation Order issued by the Striking Force Commander. The seizure of air superiority is part of the classic Japanese naval doctrine.

The four assumptions outlined above were well grounded. An espionage network in the Hawaiian Islands, together with uninterdicted cable communications between Hawaii and Japan, undoubtedly aided the Japanese in establishing their basic hypothesis—namely, that the bulk of the American Pacific Fleet would be waiting at anchor at Pearl Harbor at the time of their surprise attack. The feasibility of a surprise attack prior to a declaration of war was, of course, borne out by events.

(i) Diplomatic Deception:

The operation plan providing for the outbreak of war and the attack on Pearl Harbor was published on 5 November 1941 as Combined Fleet Top Secret Operation Order No. 1, and Y day (8 December, Japanese time) was set in Combined Fleet Top Secret Operation Order No. 2 on 7 November 1941. At the same time, the Japanese envoy Mr. Kurusu was en route to Washington to join the Japanese Ambassador in conducting conversations with the American Government. On 7 November—the date that Y day was set—a “leading Japanese and reliable informant” visited the American Ambassador in Japan reportedly at the request of Foreign Minister Togo and urged repeatedly that, whether or not Japanese concessions were deemed inadequate by the United States, it was “of the highest importance that the Washington conversations be continued and not permitted to break down.”⁶⁰ These conversations were continued throughout November and until the actual launching of the surprise attack. While it is true that Operation Order No. 2, setting Y Day, was not sufficient by itself to effect the launching of the attack on that day, it was sufficient to despatch the various Japanese task forces to their scheduled pre-invasion rendezvous points (Tankan Bay in the Kuriles for the Pearl Harbor Striking Force; Mako in the Pescadores for the main body of the Philippines Force, etc.) and have the Fleet complete its basic preparations for an attack on that day. And it seems evident, from a study of such available subsidiary orders as were issued during November 1941 by certain of the task forces involved in the plan, that the supplementary orders that were to cause the task forces to move forward to their attack positions and were to specify precisely the time for the outbreak of war, were to be issued almost as inevitable concomitants of the initial decision embodied in Operation Orders Nos. 1 and 2. While a radical change in the diplomatic situation might have caused the plan to be abandoned, it seems probable that nothing short of complete American acceptance of the Japanese terms in their note of 20 November 1941 would have been regarded as sufficient to cancel the plans already set in motion. In any event, the continuation of diplomatic negotiations by the Japanese after their task forces were already en route to their final goals must be deemed nothing short of deception.

(ii) Radio Silence:

The Striking Force, as it moved north to the sortie point in the Kuriles, and thence to the eastern Pacific, was operating under strict injunctions to maintain radio silence to help assure the secrecy of its movement and mission.

(iii) Radio Deception:

The 1st Combined Communications Unit was directed to maintain deceptive traffic to simulate the presence of the main strength of the Japanese Fleet in the Inland Sea. At the same time, the early December movements of the Japanese units en route to the south were not conducted under complete radio silence—possibly because the element of surprise for that part of the campaign could not be preserved by silence since Allied visual observations could be made of those movements and possibly because of the belief that the ability of Allied intelligence to trace the southward movements of the Southern Force, and only those movements, would further bolster the effectiveness of the strategic surprise desired for the operations of the Striking Force.

Composition of the Forces Attacking Pearl Harbor:

Striking Force

Commanding Officer: Commander in Chief, 1st Air Fleet—Vice Admiral Chuichi Naumo.

Battleship Division #3 (1st section) (Hiei, Kirishima)	2 battleships
Carrier Division #1 (Kaga, Akagi)
Carrier Division #2 (Hiryu, Soryu)
Carrier Division #5 (Shokaku, Zuikaku)	6 aircraft carriers
Cruiser Division #8 (Tone, Chikuma)	2 heavy cruisers
Destroyer Squadron #1 (Abukuma, 4 destroyer divisions)	1 light cruiser 16 destroyers

Plus 11 train vessels

Advance Expeditionary Fleet

Commanding Officer: Commander in Chief, 6th Fleet, Vice Admiral Mitsumi Shimizu.

(Isuzu, Tura)	2 light cruisers
(Katori)	1 training cruiser
1-class submarines (including Submarine Squadrons #1, #2, #3: (I–1,2, 3,4, 5, 6, 7, 16, 17, 18, 20, 22–24, 68, 69, 74)
Midget submarines	20 submarines
Plus 6 train vessels.	5 midget submarines

Execution of the Operation

The Striking Force assembled at Tankan Bay in Etorofu Island (Kuriles) during late November 1941, and sortied for the attack on or about 27 November, following a course to the eastward and then southward, as indicated on the captured track chart appended (see Appendix 3).⁶¹ Of the 11 train vessels allotted in the Operation Plan, reliable information suggests that only 3 tankers and 1 supply ship actually accompanied the Force. Apparently also, 3 submarines of the Advance Expeditionary Force (submarine fleet) accompanied the Striking Force—the other submarines having proceeded from the Inland Sea independently of the Striking Force. During the fast voyage to their destination 200 miles due north of Oahu, the vessels of the carrier force were kept fueled successfully, albeit with considerable difficulty in many cases. Upon arrival at their destination, about 200 miles due north of Oahu, the carriers launched their aircraft, which rendezvoused further south and then flew in for a coordinated attack. Three waves of these carrier aircraft were employed, commencing at 0747 and ending at 0936, local time; the first attack was on various airfields, followed by attacks on the warships in Pearl Harbor. Following the successful completion of these surprise attacks, the carrier force withdrew to the Empire, taking a circuitous route to Hashirajima and arriving on 23 December. En route, Carrier Division #2 (Hiryu, Soryu) and Cruiser Division #8 (Tone, Chikuma) were detached as a small reinforcement group for the Wake Island operation. Losses sustained by the Striking Force during the Hawaiian operation reportedly totalled 29 aircraft.

Until the completion of the surprise attack on Hawaii by the Striking Force, the “Advance Expeditionary Force” of submarines was under the command of the Striking Force Commander. The precise movements of the participating submarines prior to the actual attack are not clear, although some information suggests that the bulk of those units left the Empire in late November. So far as is known, the functions allotted the submarines in Operation Order No. 1 were carried out as planned, viz.:

(a) Until X–3, submarines were to reconnoiter important points in the Aleutians, Samoa, Fiji, and Tutuila, and were to observe and report on any powerful American forces discovered.

(b) One element was to patrol along the route of the Striking Force in advance of the movement of that Force—to ensure the undetected approach of the carrier group to its destination.

(c) Until X–5, the remaining submarines were to surround Hawaii at extreme range while one element approached and reconnoitered without being observed.

(d) On X day, the function of the submarines would be to “observe and attack the American Fleet in the Hawaii Area; make a surprise attack on the channel leading into Pearl Harbor and attempt to close it; if the enemy moves out to fight he will be pursued and attacked.”

Prior to the attack on X Day, the force of I-class submarines took up scouting positions in several allotted patrol sectors covering the waters in the vicinity of Pearl Harbor, while the 5 midget submarines were launched as a Special Attack Force to conduct an offensive attack against American warships within the Harbor and to prevent the escape of the fleet through the Harbor entrance during the scheduled aerial strike. Available data indicates that only two of the five subs penetrated into the Harbor; none of the five inflicted any damage on American units, and none of the five rejoined the Japanese Fleet. American naval units accounted for 3, possibly 4, of the total, and the 5th beached itself at Oahu.

During and after the attack, submarines outside of the Harbor area remained on patrol to oppose any possible sortie of American warships. The following is a translation of an extract from the “Report of the I–69’s Operations off Pearl Harbor, 8–10 December 1941” (Tokyo time), written by the Commanding Officer of the I–69 (flagship of Submarine Division 12) and presents a picture of the operation as experienced by one of the Japanese units on patrol:

“During daylight on December 8 (December 7 Honolulu time), the I–69 was cruising submerged, engaged in surveillance in Scouting Sector D (about 17 miles southwest of Pearl Harbor). Immediately after the attack upon the enemy by the Striking Force and the Special Attack Force, we were able to hear easily, by means of submarine sound detectors, explosions of bombs and torpedoes, and upon hearing the sound of depth charges, I judged that the Special Attack Force was engaged in heavy fighting.

“At 1400 radio orders from the Commander of Submarine Forces were received, assigning the I–69 to a surveillance in the central sector of E Inner Scouting Area (a circle with a radius of 8.5 nautical miles, with Pearl Harbor as the center). The orders further specified the annihilation of the remaining enemy forces.

“After sunset we surfaced, in an attempt to assist as much as possible, but during the night we sighted five destroyers. While submerging, we received a close-range depth-charge attack.…⁶² At 0015 on the 9th Tokyo time) we surfaced, making certain that no enemy patrol boats were within close range. On a course obliquely to the west, and at battle speed, we hurriedly recharged batteries.

“We sighted 2 patrol vessels on our port quarter and beam respectively, each about 5 kilometers away, and in about 30 minutes we reversed course. Pearl Harbor shone red in the sky, like a thing afire. It was already dawn.”

In the post-attack phase, the I–class submarines maintained their patrols for some time, and at least one of the group—the I–7—launched its aircraft to conduct a reconnaissance of Pearl Harbor to ascertain the status of the American fleet and installations. The operation plan had provided, in the event of the virtual destruction of the American Fleet at Pearl, that one Submarine Division or less would be placed between Hawaii and North America to destroy sea traffic and in fact at least one submarine—the I–17—was despatched for the Oregon coast about 14 December.

49. Undated and unsigned but stamped January 22, 1947, by the Office of the Secretary, Department of State. A notation on the original reads: “Based upon information obtained subsequent to 7 December 1941”.
50. For text, see Pearl Harbor Attack: Hearings before the Joint Committee on the Investigation of the Pearl Harbor Attack, 79th Cong., 1st sess., pt. 13, pp. 431 ff.
51. Not found in Department files.
52. Adm. Isoroku Yamamoto.
53. See Appendix 1. [Footnote in the original; appendix not printed.]
53. Capt Kameto Kurojima and Rear Adm. Seiicho Ito, respectively, of the Combined Fleet.
54. See Appendix 2 for allocation of Japanese Task Forces. [Footnote in the original; appendix not printed.]
54. Vice Adm. Chuichi Nagumo.
55. Vice Adm. Mitsumi Shimizu.
56. Vice Adm. Shigeyoshi Inoue.
57. Vice Adm. Nobutake Kondo.
58. Vice Adm. Boshiro Hosokaya.
59. Adm. Isoroku Yamamoto.
60. See memorandum of November 7, 1941, by the Ambassador in Japan, Foreign Relations, Japan, 1931–1941, vol. ii, p. 705.
61. Not attached to file copy of this document.
62. Omission indicated in the original.