The strategic importance of IWO JIMA can hardly be emphasized. Its value as an airbase midway between the MARIANAS and the heart of JAPAN has already been proved. After the occupation of SAIPAN, the Japs must have known that IWO would be attacked. The only question was WHEN. The story of their haste in constructing defenses is told in aerial photographs from 15 June 1944 until D-Day, 19 February 1945.

At the time SAIPAN was invaded, only AA defenses, hasty fire trenches, and preliminary beach defenses had been prepared at IWO JIMA. By September 1944, positions were then in evidence, and concrete pillboxes and covered artillery emplacements were under construction. By December, most beach positions were completed and had been strengthened by the construction of heavy concrete blockhouses. A cross-island defense line in depth protecting the northern part of the island was springing up along the high ground north of Airfield No. 1. Additional AA defenses increased numbers of covered artillery and coast defense guns, and more and more concrete positions and minefields on the beaches were noted on each succeeding photographic sortie. By D-Day, key positions had been connected with pillboxes and were protected by covering fire from artillery, anti-tank guns, and mortars, previously sighted in.

Intelligence prior to the invasion of IWO indicated a formidable garrison force with many major caliber weapons and extensive tank and personnel obstacles; however, aerial photographs and maps gave only a limited picture of one of the enemy’s best defensive weapons, the terrain. An amphibious assault in force was limited to the east and west beaches on the narrow southern part of the island. These conditions gave the Japs an advantage in preparing defensive positions for all-around security. Only small groups were needed to cover very narrow beaches with steep exits along the northern coast.

Captured documents, confirmed by ground study, indicate the enemy’s battle strength was conserved by countermeasures against shelling and bombing. The terrain was well suited for cover, and caves were so numerous in some sections of the island that it was impossible to plot them all on a 1:10,000 map. The course, loose, volcanic ash kept damage from shell fragmentation to a minimum. Ammunition, stores, and personnel were dispersed in caves and behind protective terrain where naval gunfire and artillery could not reach them.

Camouflage encountered and camouflage discipline during construction, as evaluated from aerial photographs, was excellent. Positions were at all times covered with materials which blended into the surrounding terrain and vegetation. Spoil from excavations was planted with grass to obliterate traces of military activity. Natural vegetation was used to a large extent to conceal well-built positions from aerial attack. While such practices should not always elude photographic interpretation, nevertheless many positions were not detected until gunfire had blown away the concealing vegetation. Maximum use was made of IWO’s sparse growth for concealing trenches, rifle pits, machine gun and artillery emplacements. Positions as large as those housing 120 mm Howitzers were undetected under the protective covering of trees in their natural state.

Coast defense artillery ranging from 4.7" (120 mm) to 6" (155 mm) guns was initially effective against our ships. All CD guns were casemated in four to six feet of reinforced concrete. Each emplacement was so positioned in the terrain that it was normally protected from naval gunfire. A ship taking a CD gun under direct fire necessarily exposed itself to the gun at which it was firing. Many guns were so camouflaged that their location was unknown until they opened fire.

Antiaircraft batteries were numerous, including 120 mm DP, 100 mm, 75 mm, and 25 mm in triple, twin, and single mounts. The Japs could not satisfactorily conceal these weapons, but their continued use against aircraft and our personnel ashore testifies to the sound construction of their emplacements.

The elaborate trench system facing the eastern beaches had been entirely abandoned. The loose nature of the sand suggests their presence as dummies, although they may have been prepared originally as hasty beach defenses before adequate concrete positions were built. Well-constructed blockhouses encasing 25mm machine canons to 120 mm short naval guns were the first defenses encountered on and near the beach. Some concrete pillboxes and sandstone revetted rifle pits gave infantry protection to the heavier weapons. The ground at the base of Airfield No. 1 was more suitable for dug in positions, and most of the infantry positions protecting the east beach were here. The positions were not so much the target for naval gunfire, and excellent observation and good fields of fire were afforeded here. The position of Airfield No. 1 limited the depth of these positions to approximately 500 to 700 yards. Supporting fires were delivered from the flanks using the observation provided by Mt. SURIBACHI and the high ground over the East Boat Basin and around Airfield No. 2. The area between the beach and Airfield No. 1 was steep and impossible for wheeled vehicles to traverse. Movement of infantrymen and tanks was impeded by the loose soil with the result that they offered excellent targets to anti-tank and anti-personnel guns and tactics.

The defenses of the West Beaches were probably more extensive and elaborate than East Beach defenses. Airfield No. 1 permitted these defenses to be 700 to 1400 yards deep giving considerably longer fields of fire and permitting a more elaborate system of dummy positions fronting the actual main positions and designed to draw our preliminary bombardment. The terrain was much more substantial for caves and underground shelters, and positions were better protected from our naval gunfire.

In the south a cross-island defense line was constructed on the terraced slopes which guarded the approaches to Mt. SURIBACHI. It consisted of a maze of concrete pillboxes, bunkers, shelters, and blockhouses which were capable of all-around defense and were mutually supporting. In addition to this elaborate surface organization, there was an equally elaborate subsurface organization of shelter, passageway, living quarters, storage areas, and cave-type firing positions which provided adequate protection against the heaviest of bombs and shells. The excellent observation from the heights of SURIBACHI and the high ground around Airfield No. 2 were the key to controlled artillery and mortar fire.

Since landing in strength were limited to the east and west beaches near the southern end of the island, the enemy prepared his main defense in depth in a cross-island belt. Beginning in the west at the rocky cliffs tot he north of the western beaches, it stretched across the island to skirt the southern end of Airfield No. 2, and terminated in the cliffs which format the northern end of the eastern beaches. The center of this belt was 2600 yards deep. It gained its strength from its depth and its concrete and steel structures, positioned so as to obtain long fields of fire which were carefully calculated and tied into the overall defense system. Reliance on caves as a shelter and a fighting position was increased here. Most of the positions showed excellent engineering and terrain appreciation. Communications were maintained by trenched wire, radios, and a labyrinth of underground tunnels connecting all areas. One of these tunnels was explored for 800 yards, and 14 entrances were found; it housed two battalion command posts and was equipped with lights and telephones. Artillery, including heavy mortars and rockets in and behind this defense belt, could cover most of the beach area. Positions were provided with complete preregistration data.

Strong points were established around commanding ground. Concrete-faced caves and infantry positions in the erosion-made crevices were frequently so close together that an equally strong defensive position existed only a few yards to the rear of flank. Contact became so close that safety of our own troops prevented use of artillery, naval gunfire, or air support in reducing these positions. One bluff, forming something similar to an amphitheater, contained two terraces and three tiers of concrete pillboxes and caves.

North of this cross-island defensive sector, the extremely rough terrain from the coast to 2000 yards inland created a natural defensive area. The number of caves and terrain characteristics somewhat compensated for the reduced amounts of concrete and steel. These features, coupled with the masses of men employed, tended to make this final defensive area equally as strong as the main defensive positions. In attacking these positions no Japs were to be seen, all being in caves or crevices in the rocks and so dispersed to give an all-around interlocking defense to each small compartment. Attacking troops frequently were subjected to fire from flanks and rear more than from their front. It was always very difficult and frequently impossible to locate exactly where defensive fires originated.

In defending IWO JIMA, the Japs employed one basic tactic which in a sense was a departure from the Japanese defensive operations hitherto generally encountered. This tactic was simply to occupy previously determined D-Day positions and maintain them, without recourse either to costly "main effort" counterattacks or organized withdrawals. This plan was not only simple in conception but in general skillfully executed and well adapted to the terrain of IWO JIMA. There were no organized attempts made to counterattack our beachhead, no large scale night counterattacks, no "all-out" banzai charge. Instead, the enemy committed a minimum number of troops to the southern beach area and defended it by delivering heavy volumes of fire from both SURIBACHI and the north so that even when the southern area was finally taken, the bulk of the enemy’s forces remained intact and were well entrenched in the most heavily fortified part of the island. The enemy, by continuing to follow his simple but basic tactic of occupying a position and refusing to yield until dug our or killed without counterattacking and without withdrawing, was able to maintain organized resistance for over twenty days. There were no tactical withdrawals, no retrograde of delaying actions in the military sense, though in some instances isolated withdrawals were made to preserve units and individuals threatened with inevitable destruction. It is now known that this defense of holding to the end without counterattack or withdrawal was the express plan conceived by the Commanding General. It was this simple tactic, coupled with the incredible rocky terrain and the maximum use of the enemy had made of this terrain in constructing fortified positions whi8ch made the capture of IWO JIMA so difficult.

Captured battle plans indicated the enemy’s fear of our tanks. Mine fields, magnetic mines, and explosives carried by hand were employed at the beach where the loose sand serioulsy impeded the movement of even full-tracked vehicles. Anti-tank guns, mainly the high velocity 75 mm and 47 mm, were the principal weapons employed inland. Many anti-tank guns had one or more supplementary or alternate positions. These weapons were sited in terrrain affording fields of fire covering possible tank routes from the beach; however, considerable empahasis appears to have been placed on locating an emplacement where it was procted by an abutting bluff or terrace, which shielded it from frontal flat trajectory fire.

Blockhouses and pillboxes near the beaches on the more open terrain were almost invariably sited for flanking fire. Wherever possible, a pillbox was behind a natural mound of sand. It often had only a small fire port which allowed approximately 30° of traverse; however, there was a sufficient number of mutually supporting pillboxes to offset the restricted field of fire of each weapon.

Many blockhouses near the beach had sand piled as high as 50 feet in fron of them. A narow firfe lane through the sand revealed the direction of fire, but many positions were so well protected that they were still firing until reducded by infantry. Pillboxes were protected similarly; infantry commanders often found it necessary to reaquest main battery fire fromj navel vessels when secondary battery fire could not blast the sand from the front of well built positions.

Relatively few tanks, mediuim and light, were present on IWO JIMA. The rough terrain, size of the island, and fixed nature of defenses probably account for the limited number. Their manner of employment was as an anti-tank weapon. They were used in support of the main cross-island defense belt either in a revetment of behind protective terrain, camouflaged and stationary. Their 37 mm, and 47 mm, and 57 mm guns fired anti-tank and personnel missions like the many 47 mm guns. Reports indicate small tank units were to be employed in support of local counterattacks. There is little evidence they were so used. The torn up condition of roads and constant artillery fire may well have prevented any movement.