In anticipation of the Battle of Iwo Jima, Lieutenant General Tadamichi Kuribayashi prepared a defense that broke with Japanese military doctrine. Rather than defending the beaches, Kuribayashi devised a defense that maximized enemy attrition. The American plan of attack was made in anticipation of a standard defense.
Japanese planning Even before the fall of Saipan in June 1944, Japanese planners knew that Iwo Jima would have to be reinforced significantly if it were to be held for any length of time, and preparations were made to send sizable numbers of men and quantities of materiel to that island. In late May, Lieutenant General Tadamichi Kuribayashi was summoned to the office of the Prime Minister, General Hideki Tōjō, and told that he had been chosen to defend Iwo Jima to the last. Kuribayashi was further apprised of the importance of this assignment when Tojo pointed out that the eyes of the entire nation were focused on the defense of Iwo Jima. Fully aware of the implications of the task, the general accepted, and by 8 June 1944, Kuribayashi was on his way to convert Iwo Jima into an impregnable fortress.
When he arrived, some 80 fighter aircraft were stationed on Iwo Jima, but by early July only four remained. A United States Navy force then came within sight of the island and bombarded it for two days, destroying every building and the four remaining aircraft.
Much to the surprise of the Japanese garrison on Iwo Jima, there was no American attempt to invade the island during the summer of 1944. There was little doubt that in time the Americans would attack, and General Kuribayashi was more determined than ever to exact the heaviest possible price for Iwo Jima, although the lack of naval and air support meant that Iwo Jima could not hold out indefinitely against an invader with sea and air supremacy.
By late July, Kuribayashi had evacuated all civilians from the island. Lieutenant General Hideyoshi Obata, commanding general of the 31st Army, early in 1944 had been responsible for the defense of Iwo Jima prior to his return to the Marianas. Following the doctrine that an invasion had to be met practically at the water's edge, Obata had ordered the emplacement of artillery and the construction of pillboxes near the beaches. General Kuribayashi had a different strategy. Instead of attempting to hold the beaches, he planned to defend them with a sprinkling of automatic weapons and infantry. Artillery, mortars, and rockets would be emplaced on the foot and slopes of Mount Suribachi, as well as in the high ground to the north of Chidori airfield.
The reason for Kuribayashi's departure from the water's edge defense strategy, which had been the normal practice for the Japanese Imperial Army, was that he predicted that American air and naval bombardments would destroy any defenses on the beaches. It had been used at Saipan to great losses for the Japanese. For water's edge defense to work, it needed support from the air and sea, none of which the Japanese Imperial Navy was capable of mounting at this point anymore. However, other military branches, especially the navy, were still insistent on the water's edge defense and demanded that Kuribayashi see to it. In the end Kuribayashi had some pillboxes built at the beach as a token measure. The pillboxes were destroyed by American bombardment.
Caves, bunkers and tunnels
A prolonged defense of the island required the preparation of an extensive system of caves and tunnels, for the naval bombardment had clearly shown that surface installations could not withstand extensive shelling. To this end, mining engineers were dispatched from Japan to draw blueprints for projected underground fortifications that would consist of elaborate tunnels at varying levels to assure good ventilation and minimize the effect of bombs or shells exploding near the entrances or exits.
At the same time, reinforcements were gradually beginning to reach the island. As commander of the 109th Infantry Division, General Kuribayashi decided first of all to shift the 2nd Independent Mixed Brigade, consisting of about 5,000 men under Major General Kotau Osuga, from Chichi to Iwo Jima. With the fall of Saipan, 2,700 men of the 145th Infantry Regiment, commanded by Colonel Masuo Ikeda, were diverted to Iwo Jima. These reinforcements, who reached the island during July and August 1944, brought the strength of the garrison up to approximately 12,700 men. Next came 1,233 men of the 204th Naval Construction Battalion, who quickly set to work constructing concrete pillboxes and other fortifications.
On 10 August 1944, Rear Admiral Rinosuke Ichimaru reached Iwo Jima, shortly followed by 2,216 naval personnel, including naval aviators and ground crews. The admiral, a renowned Japanese aviator, had been crippled in an airplane crash in the mid-twenties and, ever since the outbreak of the war, had chafed under repeated rear echelon assignments.
For the remainder of 1944, the construction of fortifications on Iwo also went into high gear. The Japanese were quick to discover that the black volcanic ash that existed in abundance all over the island could be converted into concrete of superior quality when mixed with cement. Pillboxes near the beaches north of Mount Suribachi were constructed of reinforced concrete, many of them with walls four feet thick. At the same time, an elaborate system of caves, concrete blockhouses, and pillboxes were established. One of the results of American air attacks and naval bombardment in the early summer of 1944 had been to drive the Japanese so deep underground that eventually their defenses became virtually immune to air or naval bombardment.
While the Japanese on Peleliu Island in the Western Carolines, also awaiting American invasion, had turned the improvement of natural caves into an art, the defenders of Iwo Jima developed it into a science. Because of the importance of the underground positions, 25% of the garrison was detailed to tunneling. Positions constructed underground ranged in size from small caves for a few men to several underground chambers capable of holding 300 or 400 men. In order to prevent personnel from becoming trapped in any one excavation, the subterranean installations were provided with multiple entrances and exits, as well as stairways and interconnecting passageways. Special attention had to be paid to providing adequate ventilation, since sulphur fumes were present in many of the underground installations. Fortunately for the Japanese, most of the volcanic stone on Iwo was so soft that it could be cut with hand tools.
General Kuribayashi established his command post in the northern part of the island, about 500 m northeast of Kita village and south of Kitano Point. This installation, 20 m underground, consisted of caves of varying sizes, connected by 150 m of tunnels. Here the island commander had his own war room in one of three small concrete enclosed chambers; the two similar rooms were used by the staff. Farther south on Hill 382, the second highest elevation on the island, the Japanese constructed a radio and weather station. Nearby, on an elevation just southeast of the station, an enormously large blockhouse was constructed which served as the headquarters of Colonel Chosaku Kaidō, who commanded all artillery on Iwo Jima. Other hills in the northern portion of the island were tunnelled out. All of these major excavations featured multiple entrances and exits and were virtually invulnerable to damage from artillery or aerial bombardment. Typical of the thoroughness employed in the construction of subterranean defenses was the main communications center south of Kita village, which was so spacious that it contained a chamber 50 m long and 20 m wide. This giant structure was similar in construction and thickness of walls and ceilings to General Kuribayashi's command post. A 150 m tunnel 20 m below the ground led into this vast subterranean chamber.
Perhaps the most ambitious construction project to get under way was the creation of an underground passageway designed to link all major defense installations on the island. As projected, this passageway was to have attained a total length of almost . Had it been completed, it would have linked the formidable underground installations in the northern portion of Iwo Jima with the southern part of the island, where the northern slope of Mount Suribachi alone harbored several thousand yards of tunnels. By the time the Marines landed on Iwo Jima, more than of tunnels had been completed.
A supreme effort was required of the Japanese personnel engaged in the underground construction work. Aside from the heavy physical labor, the men were exposed to heat from , as well as sulphur fumes that forced them to wear gas masks. In numerous instances a work detail had to be relieved after only five minutes. Renewed American air attacks struck the island on 8 December 1944 and became a daily occurrence until the actual invasion of the island. Subsequently, a large number of men had to be diverted to repairing the damaged airfields.
Artillery
Next to arrive on Iwo Jima were artillery units and five anti-tank battalions. Even though numerous supply ships en route to Iwo Jima were sunk by American submarines and aircraft, substantial quantities of materiel did reach Iwo Jima during the summer and autumn of 1944. By the end of the year, General Kuribayashi had available to him 361 artillery pieces of 75 mm or larger caliber, a dozen 320 mm mortars, 65 medium (150 mm) and light (81 mm) mortars, 33 naval guns 80 mm or larger, and 94 anti-aircraft guns 75 mm or larger. In addition to this formidable array of large caliber guns, the Iwo Jima defenses could boast more than 200 20 mm and 25 mm anti-aircraft guns and 69 37 mm and 47 mm antitank guns.
The firepower of the artillery was further augmented with a variety of rockets varying from an eight-inch type that weighed 90 kg and could travel 2–3 km, to a giant 250 kg projectile that had a range of more than 7 km. Altogether, 70 rocket guns and their crews reached Iwo Jima.
Tanks In order to further strengthen the Iwo defenses, the 26th Tank Regiment, which had been stationed at Pusan, Korea after extended service in Manchuria, received orders to head for Iwo Jima. The officer commanding this regiment was Lieutenant Colonel Baron Takeichi Nishi, a 1932 Olympic gold medallist. The regiment, consisting of 600 men and 28 tanks, sailed from Japan in mid-July on board the Nisshu Maru. On 18 July 1944, as the ship, sailing in a convoy, approached Chichi Jima, it was torpedoed by an American submarine, . Even though only two members of the 26th Tank Regiment were killed, all of the regiment's 28 tanks went to the bottom of the sea. It would be December before these tanks could be replaced. The 22 tanks which finally reached Iwo Jima included medium Type 97 Chi-Ha and light Type 95 Ha-Go tanks. Neither of these types were near comparable to the better armed and better armored M4 Sherman medium tanks fielded by the Americans.
Initially, Colonel Nishi had planned to employ his armor as a type of "roving fire brigade", to be committed at focal points of combat. The rugged terrain precluded such employment and, in the end, the tanks were deployed in static positions under the colonel's watchful eyes. They were either buried or their turrets were dismounted and so skillfully emplaced in the rocky ground that they were practically invisible from the air or the ground. The headquarters of the 26th Tank Regiment, which was located near the village of Maruman, was moved to the eastern part of the island when the battle began.
Defense planning