Background (3)
At the time of the Japanese attack on Pearl Harbor an Army force of about 3,700–3,800 men garrisoned Chichi Jima. In addition, about 1,200 naval personnel manned the Chichi Jima Naval Base, a small seaplane base, the radio and weather station, and various gunboat, subchaser, and minesweeping units. On Iwo Jima, the Imperial Navy had constructed an airfield about 2,000 yards northeast of Mount Suribachi. Initially stationed on this field were 1,500 naval aviation personnel and 20 aircraft. In the wake of the American seizure of the Marshall Islands and devastating air attacks against Truk in the Caroline Islands in February 1944, the Japanese military leadership conducted a reappraisal of the military situation. All indications pointed to an American drive towards the Marianas and Carolines. To counter such a move, they established an inner line of defense extending generally northward from the Carolines to the Marianas, and from thence to the Ogasawara Islands.

Japanese Pillbox with 120mm gun

In March 1944, the Thirty-First Army, commanded by General Hideyoshi Obata, was activated for the purpose of garrisoning this inner line. The commander of the Chichi Jima garrison was placed nominally in command of Army and Navy units in the Ogasawara Islands. Following the American seizure of bases in the Marshalls in the battles of Kwajalein and Eniwetok in February 1944, both Army and Navy reinforcements were sent to Iwo Jima. Five hundred men from the naval base at Yokosuka and an additional 500 from Chichi Jima reached Iwo Jima during March and April 1944. At the same time, with the arrival of reinforcements from Chichi Jima and the home islands, the Army garrison on Iwo Jima had reached a strength of over 5,000 men, equipped with 13 artillery pieces, 200 light and heavy machine guns, and 4,552 rifles. In addition, the defense boasted 120 mm coast artillery guns, twelve heavy anti-aircraft guns, and thirty 25 mm dual-mount anti-aircraft guns. The loss of the Marianas during the summer of 1944 greatly increased the importance of the Ogasawaras for the Japanese, who were fully cognizant that the loss of these islands would facilitate American air raids against the home islands. Such raids, beyond any doubt, would raise havoc with the entire Japanese war production program, and deal a severe blow to civilian morale. Final Japanese plans for the defense of the Ogasawaras were overshadowed by the fact that the Imperial Japanese Navy had already lost most of its naval strength and no longer constituted a major factor in frustrating possible American landings. Moreover, aircraft losses throughout 1944 had been so heavy that, even if war production was not materially slowed by American air attacks, combined Japanese air strength was not expected to increase to 3,000 aircraft until March or April of 1945. Even then, these planes could not be used from bases in the home islands against Iwo Jima because their range did not exceed 550 miles (890 km); besides, all available aircraft had to be hoarded for possible use on Taiwan and adjacent islands where land bases were available in close proximity. In the battle of Iwo Jima the Japanese only used ground units, no planes or boats of any kind were involved.

Japanese Pillbox with 120mm gun

A United States Navy force boldly appeared within sight of the island and subjected the Japanese to a naval bombardment from point-blank range over two days. This shelling destroyed every building on the island and smashed the four remaining aircraft. Much to the surprise of the Japanese garrison on Iwo, an American invasion of the island did not materialize during the summer of 1944. There was little doubt that in time the Americans would be compelled to attack the island. General Kuribayashi was more determined than ever to exact the heaviest possible price for Iwo when the invaders came. Without naval and air support, it was a foregone conclusion that Iwo could not hold out indefinitely against an invader possessing both naval and air supremacy. As a first step in readying Iwo for a prolonged defense, the island commander ordered the evacuation of all civilians from the island. This was accomplished by late July.

Next came an overall plan for defense of the island. Lieutenant General Hideyoshi Obata, Commanding General of the Thirty-First Army, early in 1944 had been responsible for the defense of Iwo prior to his return to the Marianas. At the time, faithful to 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 different ideas. Instead of a futile effort to hold the beaches, he planned to defend the latter 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. 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.

Pre-invasion Iwo with airfields

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 defenses could boast of more than two hundred 20 mm and 25 mm antiaircraft guns and 69 37 mm and 47 mm antitank guns. The fire power of the artillery was further supplemented 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. 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 for Iwo. The officer commanding this regiment was Lieutenant Colonel Baron Takeichi Nishi. The regiment, consisting of 600 men and 28 tanks, sailed from Japan in mid-July on board the Nisshu Maru. As the ship, sailing in a convoy, approached Chichi Jima on 18 July 1944, it was torpedoed by an American submarine, USS Cobia. Even though only two members of the 26th Tank Regiment were sunk, all of the regiment's 28 tanks went to the bottom of the sea. It would be December before these tanks could be replaced, but 22 finally reached Iwo Jima. 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, under the colonel's watchful eyes, the tanks were deployed in static positions. 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 from the ground.

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 was 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 developed it into a science. Because of the importance of the underground positions, 25 percent 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 warroom 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 Kaido, 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 17 miles (27 km). 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 11 miles (18 km) 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 30–50 °C (90–120 °F), 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. When renewed American air attacks struck the island on 8 December 1944 and thereafter became a daily occurrence until the actual invasion of the island, a large number of men had to be diverted to repairing the damaged airfields.

While Iwo Jima was being converted into a major fortress with all possible speed, General Kuribayashi formulated his final plans for the defense of the island. This plan, which constituted a radical departure from the defensive tactics used by the Japanese earlier in the war, provided for the following major points:

1. In order to prevent disclosing their positions to the Americans, Japanese artillery was to remain silent during the expected prelanding bombardment. No fire would be directed against the American naval vessels.

2. Upon landing on Iwo Jima, the Americans were not to encounter any opposition on the beaches.

3. Once the Americans had advanced about 500 m inland, they were to be taken under the concentrated fire of automatic weapons stationed in the vicinity of Motoyama airfield to the north, as well as automatic weapons and artillery emplaced both on the high ground to the north of the landing beaches and Mount Suribachi to the south.

4. After inflicting maximum possible casualties and damage on the landing force, the artillery was to displace northward from the high ground near the Chidori airfield.

Despite intermittent harassment by American submarines and aircraft, additional personnel continued to arrive on Iwo until February 1945. By that time General Kuribayashi had under his command a force totalling between 21,000 and 23,000 men, including both Army and Navy units. General Kuribayashi made several changes in his basic defense plan in the months preceding the American invasion of Iwo Jima. The final stratagem, which became effective in January 1945, called for the creation of strong, mutually supporting positions which were to be defended to the death. Neither large scale counterattacks, withdrawals, nor banzai charges were contemplated. The southern portion of Iwo in the proximity of Mount Suribachi was organized into a semi-independent defense sector. Fortifications included casemated coast artillery and automatic weapons in mutually supporting pillboxes. The narrow isthmus to the north of Suribachi was to be defended by a small infantry force. On the other hand this entire area was exposed to the fire of artillery, rocket launchers, and mortars emplaced on Suribachi to the south and the high ground to the north. A main line of defense, consisting of mutually supporting positions in depth, extended from the northwestern part of the island to the southeast, along a general line from the cliffs to the northwest, across Motoyama Airfield No. 2 to Minami village. From there it continued eastward to the shoreline just south of Tachiiwa Point. The entire line of defense was dotted with pillboxes, bunkers, and blockhouses. Colonel Nishi's immobilized tanks, carefully dug in and camouflaged, further reinforced this fortified area, whose strength was supplemented by the broken terrain.

Flame throwers in action at Iwo Jima video (click here).
Color Film To the Shores fo Iwo Jima from WW2 in color.
This footage showcases raw and uncut graphic footage. Viewer discretion is advised.

A second line of defense extended from a few hundred yards south of Kitano Point at the very northern tip of Iwo across the still uncompleted Airfield No. 3, to Motoyama village, and then to the area between Tachiiwa Point and the East Boat Basin. This second line contained fewer man-made fortifications, but the Japanese took maximum advantage of natural caves and other terrain features. As an additional means of protecting the two completed airfields on Iwo from direct assault, the Japanese constructed a number of antitank ditches near the fields and mined all natural routes of approach.

When, on 2 January, more than a dozen B-24 Liberator bombers raided Airfield No. 1 and inflicted heavy damage, Kuribayashi diverted more than 600 men, 11 trucks, and 2 bulldozers for immediate repairs. As a result, the airfield again became operational after only 12 hours. Eventually, 2,000 men were assigned the job of filling the bomb craters with as many as 50 men detailed to each bomb crater. The end of 1944 saw American B-24 bombers over Iwo Jima almost every night while U.S. Navy carriers and cruisers frequently sortied into the Ogasawaras. On 8 December 1944, American aircraft dropped more than 800 tons of bombs on Iwo Jima, which shook the Japanese up but did very little real damage to the island defenses. Even though frequent air raids interfered with the Japanese defensive preparations and robbed the garrison of much badly needed sleep, progress of the work was not materially slowed.

As early as 5 January 1945, Admiral Ichimaru conducted a briefing of naval personnel at his command post in which he informed them of the destruction of the Japanese Fleet at the Battle of Leyte Gulf, the loss of the Philippines, and the expectation that Iwo would shortly be invaded. Exactly one month later, Japanese radio operators on Iwo reported to the island commander that code signals of American aircraft had undergone an ominous change. On 13 February, a Japanese naval patrol plane spotted 170 American ships moving northwestward from Saipan. All Japanese troops in the Ogasawaras were alerted and occupied their battle positions. On Iwo Jima, preparations for the pending battle had been completed, and the defenders were ready.

On 7 October 1944 Admiral Chester Nimitz and his staff issued a staff study for preliminary planning, which clearly listed the objectives of Operation Detachment. The overriding purpose of the operation was to maintain unremitting military pressure against Japan and to extend American control over the Western Pacific. In American hands, Iwo Jima could be turned into a base from which to attack the Japanese home islands, protect bases in the Marianas, cover naval forces, conduct search operations of the approaches to the Japanese home islands, and provide fighter escort for very long-range operations. Three tasks specifically envisioned in the study were the reduction of enemy naval and air strength and industrial facilities in the home islands; the destruction of Japanese naval and air strength in the Bonin Islands, and the capture, occupation, and subsequent defense of Iwo Jima, which was to be developed into an air base.

On 9 October, General Holland Smith received the staff study, accompanied by a directive from Admiral Nimitz ordering the seizure of Iwo Jima. This directive designated specific commanders for the operation. Admiral Raymond A. Spruance, Commander, Fifth Fleet, was placed in charge as Operation Commander, Task Force 50. Under Spruance, Vice Admiral Richmond Kelly Turner, Commander, Amphibious Forces, Pacific, was to command the Joint Expeditionary Force, Task Force 51. Second in command of the Joint Expeditionary Force was Rear Admiral Harry W. Hill. General Holland Smith was designated Commanding General, Expeditionary Troops, Task Force 56. It was not accidental that these men were selected to command an operation of such vital importance that it has since become known as "the classical amphibious assault of recorded history." All of them had shown their mettle in previous engagements. One chronicler of the Iwo Jima operation put it in the following words: The team assigned to Iwo Jima was superb the very men who had perfected the amphibious techniques from the Battle of Guadalcanal to the Battle of Guam. Nearly every problem, it was believed, had been met and mastered along the way, from the jungles of Guadalcanal up through the Solomons, and across the Central Pacific from the bloody reefs of Battle of Tarawa to the mountains of the Marianas. The U.S. V Amphibious Corps scheme of maneuver for the landings was relatively simple. The 4th and 5th Marine Divisions were to land abreast on the eastern beaches, the 4th on the right and the 5th on the left.

Iwo was pounded for months before the invasion by the Army and Navy. However, 22,000 Japanese soldiers are buried deep inside the volcanic island thru an elaborate cave and tunnel system

On the right of the 28th Marines, the U.S. 27th Marine Regiment, under Colonel Thomas A. Wornham, was to attack towards the west coast of the island, then wheel northeastward and seize the O-1 Line. Action by the 27th and 28th Marines was designed to drive the enemy from the commanding heights along the southern portion of Iwo, simultaneously securing the flanks and rear of VAC. As far as the 4th Marine Division was concerned, the U.S. 23rd Marine Regiment, commanded by Colonel Walter W. Wensinger, was to go ashore on Yellow 1 and 2 beaches, seize Motoyama Airfield No. 1, then turn to the northeast and seize that part of Motoyama Airfield No. 2 and the O-1 Line within its zone of action. After landing on Blue Beach 1, the U.S. 25th Marine Regiment, under Colonel John R. Lanigan, was to assist in the capture of Airfield No. 1, the capture of Blue Beach 2, and the O-1 Line within its zone of action. The U.S. 24th Marine Regiment, under Colonel Walter I. Jordan, was to be held in 4th Marine Division reserve during the initial landings. The U.S. 26th Marine Regiment, led by Colonel Chester B. Graham, was to be released from corps reserve on D-Day and prepared to support the 5th Marine Division. Division artillery was to go ashore on order from the respective division commanders. The 4th Marine Division was to be supported by the U.S. 14th Marine Regiment, commanded by Colonel Louis G. DeHaven; Colonel James D. Wailer's U.S. 13th Marine Regiment was to furnish similar support for the 5th Marine Division. The operation was to be so timed that at H-Hour 68 Landing Vehicle Tracked, comprising the first wave, were to hit the beach. These vehicles were to advance inland until they reached the first terrace beyond the high-water mark. The armored amphibians would use their 75mm howitzers and machine guns to the utmost in an attempt to keep the enemy down, thus giving some measure of protection to succeeding waves of Marines who were most vulnerable to enemy fire at the time they debarked from their LVTs. Though early versions of the VAC operations plan had called for tanks of the 4th and 5th Tank Battalions to be landed at H plus 30, subsequent studies of the beaches made it necessary to adopt a more flexible schedule. The possibility of congestion at the water's edge also contributed to this change in plans. In the end, the time for bringing the tanks ashore was left to the discretion of the regimental commanders. The Allies wanted Iwo Jima not only to neutralize threats to its bombers and shipping, but to use its airfields for fighter escort and emergency bomber landings. On February 16, 1945, they commenced a massive three-day air and naval bombardment of the island.