A translated Japanese tratise on the tactical defense of a Pacific coral island in WWII, from Tactical and Technical Trends, No. 37, November 4, 1943. [DISCLAIMER: The following text is taken from the U.S. War Department publication Tactical and Technical Trends. As with all wartime intelligence information, data may be incomplete or inaccurate. No attempt has been made to update or correct the text. Any views or opinions expressed do not necessarily represent those of the website.]

JAPANESE DEFENSE OF A CORAL ISLAND The vastness of the Pacific with its multitudinous small islands presents many problems of defense. The following article is based on excerpts from a translated Japanese tactical treatise involving a study of the dispositions to be made for the defense of a hypothetical coral island. The study contemplates the utilization of field fortifications when landing operations are threatened by an enemy who is superior in air and sea power. There are many points which are not covered in this article, but it is being published as it is, in the hope that it will be of some value. *          *          * a. General The organization and establishment of positions differs depending on the size of the island, the garrison's forces, weapons, and materiel, the situation of beaches where it is possible for the enemy to land, and many other factors. However, in case the strength of the garrison is comparatively small in relation to the size of the island, if an attempt is made to defend the island by stationing troops all around the shore line, the defense will be weak everywhere. Therefore, it is best to organize defense areas at important places so as to hold securely the important installations, with a large force being held in reserve. The intervals between defense areas should be covered by fire and obstacles should be constructed in these intervals. If necessary, small supporting points between the main defensive areas may be occupied with a part of the forces. For obstacles within the island, it is necessary to select types of obstacles which will not show in aerial photographs so that the enemy cannot determine our dispositions for defense. The plan of defense should be to destroy the enemy at the water's edge, but should he land, he will be annihilated by counterattacks. b. Garrison of Defense Areas The garrison of the defense areas differs, depending upon the mission, the size of the area occupied, and other considerations. However, on a coral island, ordinarily from one to two companies are necessary for the garrison of a defense area. The frontage assigned varies according to the type, number, etc. of the weapons which are to be located in the defense area, but if two platoons are placed on the front line, one company would be able to hold a front of 550 meters*. If the interval between platoons is covered by the coordinated fire of heavy machine guns from the platoon areas (the interval between platoons being 500 to 600 meters), the frontage assigned to a company may be about 1,000 meters. The depth is about 150 meters for the platoon and about 400 meters for the company. c. Method of Determining Frontage The frontage assigned to the various units is determined by using density of fire as the standard. (1) Conditions Density of fire required for stopping an attack dead -- 5 shots on one meter of front every minute. Minimum limit of density of fire -- 3 shots on one meter front every minute. (These include rifle, LMG, and MG). Rate of fire -- rifle (10 shots per minute)              -- LMG (150 shots per minute) The heavy machine gun is used for flank defense. On the front, rifles, LMGs, etc., are used according to a fixed plan, or to fire at will on important targets. The degree of density, however, in each sector is not calculated. (2) Frontages Covered by Various Units (a) Squad front covered = 50 to 90 meters     11 rifles firing 10 rounds per minute = 110 rounds per minute      1 LMG firing 150 rounds per minute   = 150 rounds per minute                                     Total = 260 rounds per minute     260 divided by 5 shots per meter per minute = 50 meters     260 divided by 3 shots per meter per minute = 90 meters Therefore, 50 to 90 meters frontage. (b) Platoon front covered = 200 to 300 meters       3 rifle squads on front line - 50 x 3 = 150 meters                                      90 x 3 = 270 meters     With heavy grenade squads on front line = 200 to 300 meters (c) Company front covered = 400 to 600 meters     Two platoons on front line - 200 x 2 = 400 meters                                  300 x 2 = 600 meters Therefore, 400 to 600 meters frontage. (d) Battalion front covered = 800 to 1,800 meters     2 companies on the front line -         400-600 meters x 2 = 800 to 1,200 meters     3 companies on the front line -         400-600 meters x 3 = 1,200 to 1,800 meters Therefore, 800 to 1,800 meters frontage. d. Frontage Depending on Degree of Deployment (1) Front occupied by squad--about 35 meters: Interval between men in deployment is 6 paces (about 4 to 5 meters). In building earthworks the interval between the fox holes of the riflemen is about 3 to 4 meters, and as one squad has about 10 men, one squad's frontage is 30 to 40 meters. (2) Interval between squads--normally about 65 meters: The frontage which one artillery battery can cover with an even density of fire is about 100 meters, so it cannot support two squads. Therefore, if the squad's front is 35 meters, the interval between squads should be about 65 meters. In such a situation, it is necessary to consider laying down barrages in the intervals between the squads by the artillery in the rear. (3) Distance between the squad in a platoon position--about 100 meters: The distance should be about 100 meters for the same reasons as those given in the preceding paragraph. (4) Area occupied by the platoon (see figure 1.) The area occupied by the platoon has a frontage of approximately 230 meters and a depth of approximately 100 meters. The depth may be greater depending on the position of the platoon leader, the location of the heavy weapons, etc. (5) Area occupied by the company (see figure 2.) The area occupied by the company has a frontage of approximately 550 meters and a depth of approximately 400 meters. (6) Area occupied by the company when a wide front is assigned and the interval between the front line platoons is covered by machine guns (see figure 3.) When there is a wide company front and the interval between the front line platoons is covered by machine guns the company frontage will be approximately 1,000 meters and the depth approximately 400 meters. (7) Area occupied by the battalion (see figure 4.) With two companies in the front line the area occupied by the battalion will have a frontage of 1,200 to 1,400 meters and a depth of approximately 1,100 meters. When there are three companies in the front line the battalion area will have a frontage of 1,800 to 2,200 meters and a depth of approximately 1,100 meters. In the above, a standard has been sought, but in actual practice, circumstances will vary so much that it is not possible to follow this standard rigidly. In case there is much dead space, the capabilities of the weapons cannot be fully utilized and the frontage is very much reduced. In the case of firing over water as in coastal defense, the frontage can be increased up to the limit of easy control. e. Reserves The reserves should be stationed at a point from which it is easy to move them up to reinforce the front line, or to counterattack, as required by the situation. On coral islands, there is little space in which to maneuver for a counterattack, so in many cases counterattacks must be made frontally. Therefore, it is very advantageous to have tanks, armored cars, and so on, available. Furthermore, as the enemy has planned to use amphibious tanks in landing, it is necessary to have material ready for close-quarter antitank attacks. f. Interval Between Defense Areas The interval between defense areas will vary with the type of weapons used to cover these intervals. In case machine guns are to cover the space with cross-fire from both sides, the interval is approximately 600 meters; in case machine guns from each side cover one half of the ground, it may be about 1,000 meters. When covered by artillery fire, the interval may be from 2,000 to 4,000 meters. However, against coastal defenses, the enemy often tries surprise night attacks, so the interval between defense areas must be decreased and the defenders must endeavor to destroy the enemy at the water's edge. Therefore, the ability of the machine gun to cover the interval should be the limiting factor for the size of the interval, and it should not be extended much beyond 1,000 meters. The enemy's landing must be prevented by blocking the intervals between defense areas with obstacles as well as by covering it with fire. Wire entanglements are commonly used as obstacles, and in areas where it would be easiest for the enemy to land, wire entanglements and land mines are used together. Charged wire entanglements can be used effectively. In case an interval between defense areas is very great, it is necessary to close the gap by organizing small support areas between the defense areas. g. Location of Machine Guns The essential points to be considered in emplacing machine guns to deliver flanking fire are as follows: (1) Locate machine guns to be used to deliver flanking fire upon the attackers so that it cannot be seen from the front, in a covered position if necessary, and so that the loophole cannot be fired on, thus preventing the enemy from advancing upon the position. (2) Covered machine-gun positions whose loopholes can easily be seen and which are destroyed early are of no value. In such cases, it is best to place the machine gun in an open emplacement in a rifle position. h. Disposition of Artillery, AA Guns and Other Equipment The heavy and medium caliber guns are chiefly used in shelling the ships covering the enemy's landing, the transports, etc., and the small caliber guns are to destroy the enemy's landing boats while off shore or, when he lands, to cooperate in the fight at the water's edge. They also have the duty of engaging the enemy's tanks. Usually the heavy and medium caliber guns are placed at the most important points, and the small caliber guns are distributed among the defense areas. All of these guns must be fully protected by the infantry in the defensive areas. The small caliber guns, however, when necessary, may occupy positions outside defense areas, or a part of them may be held in reserve. It is essential that other important equipment also be placed within the defense areas and thoroughly protected. i. Location of Observation Posts Since the enemy plans to approach at high speed at night with highly maneuverable ships to make a surprise landing, it is necessary to keep an especially strict guard to avoid being surprised. To this end it is necessary that the observation net be organized so that important areas can be observed from several directions and so that even the comparatively unimportant areas will not be unwatched. j. Measures to be Taken Against Bombing and Shelling from Enemy Ships Coral islands are generally level and it is difficult to utilize the terrain for the protection and concealment of positions and installations. It is most necessary to use camouflage to conceal the positions and the disposition of weapons, etc., and to use dispersal to limit damage. Of course, it is to be expected that there will be strong, permanent installations built to stand up under shelling and bombing, but these cannot be built to meet an emergency. Ones which are strong enough to be proof from bullets and shell fragments may be considered advantageous. Furthermore, the heavy weapons, etc., used for flank defense should have light covers; the other weapons should be uncovered, and completely camouflaged. At the same time reserve positions should be established and, emplacements for the weapons should be constructed near the position to prevent damage during shellings and bombing. *To convert meters to yards add roughly 10 percent to the number of meters.

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