Iwo Jima – the penultimate step in the US advance to the Japanese home islands

On 19 February 2011 it will be the 76^th anniversary of Operation ‘Detachment’, which was the US strategic undertaking to take and hold the tiny island of Iwo Jima, which was seen as the last but one step in the advance of the US Army and US Navy’s forces north toward what was expected to end with the amphibious landings on and conquest of the Japanese home islands from a time late in 1945.

‘Detachment’ was completed between 19 February and 26 March 1945. The importance of this undertaking lay in the fact that Iwo Jima’s capture provided the US forces with two completed airfields and one incomplete airfield. These could then be brought into service for emergency landings by Boeing B-29 Superfortress heavy bombers damaged over Japan and unlikely to make it back to their main bases in the Mariana islands, which had been seized by US forces in ‘Forager’ between 15 June and 10 August 1944, and could also serve as the bases for long-range fighters. These could therefore escort the bombers operating from the large bases on the Mariana islands. At the psychological level the operation also signalled to the Japanese the fact that the US forces were still driving forward on Japan and that the island’s loss was a clear sign of inevitable defeat, with only Okinawa left between the advancing Americans and the home islands.

Iwo Jima is one of the Kazan Retto (Volcano islands), the southernmost part of the Ogasawara Gunto (Bonin islands), which lies about 670 miles (1080 km) to the south of Tokyo and 625 miles (1130 km) to the north of Saipan in the Marianas, and thus almost exactly half-way between Tokyo and Saipan. On its north-east/south-west axis the island is less than 5 miles (8 km) long, and its width varies from about 2.5 miles (4 km) in the north to a mere 0.5 mile (0.8 km) in the south. The island has an area of less than 8 sq miles (20.7 km²), relies on trapped rain for its fresh water, and in its simplest terms is a very rugged plateau dominated at its northern end by a mass of low but sharp ridges and ravines, and at its southern tip by Mt Suribachi, the 546-ft (166-m) stump of an extinct volcano and the island’s highest point. At the time of the Japanese attack on Pearl Harbor in December 1941, a Japanese army force of some 3,700 to 3,800 men garrisoned Chichi Jima, the largest of the Ogasawara Gunto’s islands, and the Japanese navy had some 1,200 men at the Chichi Jima naval base, a small seaplane base, a radio and weather station, and various gunboat, submarine chaser and minesweeper units. On Iwo Jima, the Japanese navy had constructed an airfield about 2,000 yards (1830 m) north-east of Mt Suribachi. Initially stationed on this field were 1,500 Japanese navy air force personnel and 20 aircraft.

In the wake of the American seizure of the Marshall islands, starting with ‘Galvanic’ in November 1943, and the devastating ‘Hailstone’ air offensive against the Japanese naval strength located at Truk in the Caroline islands during February 1944, the Japanese high command completed a major reassessment of Japan’s strategic situation. It seemed clear that there would be US offensives against the Mariana and Caroline island groups, which were in fact undertaken as ‘Forager’ and ‘Stalemate II’ in July and September 1944 respectively. To counter such a move, the Japanese decided to create an inner defence perimeter extending basically north from the Caroline islands to the Mariana islands, and thence to the Ogasawara Gunto. In March 1944 General Hideyoshi Obata’s 31^st Army, headquartered on Truk island, was activated to garrison this inner defence line, and the commander of the Chichi Jima garrison was placed in nominal command of the Japanese army and navy units in the Ogasawara Gunto. Following the American seizure of bases in the Marshall islands, in the ‘Flintlock’ and ‘Catchpole’ battles for Kwajalein and Eniwetok atolls respectively, during February 1944 the Japanese army and navy each sent reinforcements to Iwo Jima. Some 500 men from the naval base at Yokosuka and an additional 500 from Chichi Jima arrived during March and April 1944, and at the same time the arrival of reinforcements from Chichi Jima and the home islands boosted the strength of the Japanese army’s garrison on Iwo Jima to more than 5,000 men with 13 pieces of artillery and 200 machine guns. The defence also boasted 120-mm (4.72-in) coast defence guns, 12 heavy anti-aircraft guns, and 30 25-mm twin anti-aircraft cannon.

The loss of the Mariana islands during the summer of 1944 greatly increased the importance of the Ogasawara Gunto and Kazan Retto to the Japanese, who were only too aware that the loss of these islands would enhance the efficacy of US air raids against the home islands as, from bases in the Mariana islands, the American bomber offensive would be considerably more effective than that flying from Chinese bases. The final Japanese plans for the defence of the Ogasawara Gunto and Kazan Retto were overshadowed by the fact that the Japanese navy had already lost most of its naval strength, especially in the devastatingly unsuccessful ‘A’ and ‘Sho’ operations off the Marianas and Philippine islands respectively, and could therefore no longer be seen as a significant element in any Japanese plan to defeat American landings. Moreover, Japanese aircraft losses throughout 1944 had been so heavy that, even without the increasing effect of the US bombing campaign, the overall strength of the Japanese air forces was not expected to increase to 3,000 aircraft until March or April 1945. Even then, these aircraft could not be used from bases in the home islands against US forces based on a captured Iwo Jima because their range did not exceed 550 miles (890 km). Moreover, all available aircraft had to be retained for possible use on Taiwan and other nearby islands off the coast of China, where land bases were available in close proximity to any fighting which might start. Thus in the battle of Iwo Jima the Japanese used only ground units.

The defence of Iwo Jima was based on the knowledge that victory on the island was impossible, and that the island should therefore be held for as long as possible to increase the time available for the preparation of an effective defence of the Japanese home islands, and even kamikaze air attacks, surprise attacks by submarines, and coup-de-main operations by parachute units were seen only within this context. In the first weeks of 1945, Japan faced the prospect of invasion by Allied forces. On a daily basis ‘Scavenger’ bomber raids from the Mariana islands were striking targets in the Japanese home islands, and in the battle to check if not defeat this US offensive Iwo Jima was a vital early warning station from which radio reports of inbound bomber formations could be sent to the home islands, thus facilitating the task of the Japanese army and navy air forces in getting adequate numbers of fighters into the air above the anticipated targets. At the end of the ‘King II’ offensive which took Leyte island in the Philippine islands by the end of December 1944, the Americans had a two-month interval in their operational schedule before the planned ‘Iceberg’ invasion of Okinawa and, deemed unacceptable, this lull was filled by the ‘Detachment’ capture of Iwo Jima.

Even before the fall of Saipan in July 1944, Japanese planners knew that Iwo Jima would have to be reinforced materially if it was to be held for any useful time, and preparations had then been made to send sizeable personnel and matériel reinforcements to the island. Late in May, Lieutenant General Tadamichi Kuribayashi was given the task of holding Iwo Jima, and by 8 June was on his way to convert Iwo Jima into a fortress which could withstand any type of attack for an extended period. When Kuribayashi reached his new command, some 80 fighter aircraft were stationed on Iwo Jima, but by a time early in July there were just four left. Then a US Navy force approached the island and subjected it to a two-day gunfire bombardment, which destroyed every building and smashed the four remaining aircraft. Much to the surprise of the Japanese garrison, a US invasion did not take place during the summer of 1944. Even so, no one had any doubt that in time the Americans would indeed launch an invasion. As a first step in readying Iwo Jima, Kuribayashi ordered the evacuation of all civilians, an exodus completed late in July. Next came an overall plan for defence of the island. Up to the time of his return to the Mariana islands, Obata had adhered to the well established Japanese tactical doctrine that an amphibious invasion had to be beaten back right on the water’s edge, and had accordingly ordered the emplacement of artillery and the construction of pillboxes dominating Iwo Jima’s beaches. Kuribayashi had different ideas: he felt that any effort to hold the Americans right on the beaches would be futile, and therefore planned to screen the beaches with only modest numbers of automatic weapons and infantry, while his more effective weapons (artillery, mortars and rockets) were emplaced on the foot and slopes of Mt Suribachi, as well as in the high ground north of Chidori Airfield No. 1.

Kuribayashi also believed that any prolonged defence of the island had to be based on an extensive system of caves and tunnels, for the naval bombardment had clearly shown that surface installations could not withstand prolonged shelling. To this end, mining engineers were dispatched from Japan to plan a complex of underground fortifications based on an elaborate network of tunnels at varying depths to assure good ventilation and minimise the effect of bombs or shells exploding near the entrances or exits. Over this period the island was gradually being reinforced. As commander of the 109^th Division, Kuribayashi decided first of all to shift Major General Kotoo Osuga’s 2^nd Independent Mixed Brigade, consisting of about 5,000 men (309^th to 312^th and 314^th Independent Battalions, as well as artillery, engineer and communications elements), from Chichi Jima to Iwo Jima. With the fall of Saipan, 2,700 men of Colonel Masuo Ikeda’s 145^th Regiment (four battalions) of the same division’s 1^st Mixed Brigade, were diverted to Iwo Jima. These reinforcements, which reached the island during July and August 1944, raised the strength of the army garrison to about 12,700 men. Next there arrived 1,233 members of the 204^th Naval Construction Battalion, who quickly set to work building concrete pillboxes and other fortifications. On 10 August 1944, Rear Admiral Toshinosuke Ichimaru reached Iwo Jima just before another 2,216 naval personnel including naval aviators and ground crews which were organised into combat battalions. Next came artillery units and five anti-tank battalions. Even though numerous supply ships were sunk by US aircraft and submarines while trying to reach Iwo Jima, substantial quantities of matériel reached the island during the summer and autumn of 1944.

By the end of the year, therefore, Kuribayashi had available to him 361 pieces of artillery in calibres of 75 mm (2.95 in) or greater, 12 320-mm (12.6-in) mortars, 65 150-mm (5.91-in) heavy and 81-mm (3.2-in) medium mortars, 33 naval guns in calibres of 80 mm (3.15 in) or greater, and 94 anti-aircraft guns in calibres of 75 mm (2.95 in) or greater. In addition to these large-calibre weapons, the defences of Iwo Jima had more than 200 20-mm and 25-mm anti-aircraft cannon as well as 69 37-mm and 47-mm anti-tank guns. The firepower of the artillery was further supplemented by a variety of rockets varying from a 203-mm (8-in) type weighing 198 lb (90 kg) and possessing a range of up to 3000 m (3,280 yards) to a giant 551-lb (250-kg) projectile possessing a range of more than 7000 m (7,655 yards). In total, the defences of Iwo Jima included more than 70 rocket launchers.

As a further strengthening of the defence, Lieutenant Colonel Baron Takeichi Nishi’s 26^th Tank Regiment, stationed at Pusan in southern Korea after extended service in Manchukuo, received orders to move to Iwo Jima. Consisting of 600 men and 28 tanks, the regiment sailed from Japan in mid-July on board Nisshu Maru, but as it approached Chichi Jima in a convoy on 18 July 1944, the ship was torpedoed by the US submarine Cobia. Even though only two members of the 26^th Tank Regiment were lost, all of the regiment’s 28 tanks went down with the ship. It would be December before these could be replaced, but 22 tanks finally reached Iwo Jima. Nishi had initially planned to employ his armour as a type of ‘mobile fire brigade’ for commitment at key spots and times. The rugged terrain precluded such employment, however, so finally the tanks were deployed in static positions: they were either buried whole or stripped of their turrets, which were emplaced in the rocky terrain so skilfully that they were practically invisible from the air or from the ground.

For the remainder of 1944 the construction of Iwo Jima’s fortifications was pushed forward as rapidly and extensively as possible. The Japanese quickly learned that the black volcanic ash that existed in abundance all over the island could be converted into an excellent concrete when mixed with cement. Pillboxes near the beaches north of Mt Suribachi were constructed of reinforced concrete, many of them with walls 4 ft (1.2 m) thick, and at the same time a complex of caves, concrete blockhouses and pillboxes was established. Thus 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 defences became effectively immune to air or naval bombardment. While the Japanese defenders of Peleliu island in the western part of the Caroline islands, also awaiting an American attack (‘Stalemate II’), had turned the improvement of natural caves into an art, those of Iwo Jima developed it into a science. Because of the importance of the underground positions, 25% of the garrison was detailed to tunnelling. Underground positions 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 was paid to the provision of adequate ventilation, since fumes from the islands sulphur deposits were present in many of the underground installations. Fortunately for the Japanese, most of the volcanic stone on Iwo Jima was so soft that it could be cut with hand tools. Kuribayashi established his command post in the northern part of the island, about 550 yards (500 m) north-east of Kita village and south of Kitano Point. This installation, 65 ft (20 m) underground, consisted of caves of varying sizes connected by 165 yards (150 m) of tunnels. Farther south, on Hill 382, the second highest elevation on the island, the Japanese constructed a radio and weather station. Nearby, on a rise just south-east of the station, a huge blockhouse was constructed as the headquarters of Colonel Chosaku Kaido, commanding all of Iwo Jima’s artillery. Other hills in the northern portion of the island were tunnelled out, and typical of the thoroughness employed in the construction of the subterranean defences was the main communications centre south of Kita village, which was so spacious that it contained a chamber 55 yards (50 m) long and 22 yards (20 m) wide. This major structure was similar in construction and thickness of walls and ceilings to Kuribayashi’s command post, and was accessed by a 165-yard (150-m) tunnel 65 ft (20 m) below the ground. Perhaps the most ambitious construction project to get under way was the creation of an underground passageway designed to link all the major defence positions and installations on the island. As projected, this passageway was to have attained a total length of almost 17 miles (27 km) and, had it been completed, 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 Mt Suribachi alone harboured several thousand yards of tunnels. By the time the forces of the US Marine Corps 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 labour, the men were exposed to heat from 30 to 50° C (85 to 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 US air attacks struck the island on 8 December 1944 and thereafter became a daily occurrence until the invasion started, a large number of men had to be diverted to repairing the damaged airfields. As Iwo Jima was being converted into a sturdy fortress with all possible speed, Kuribayashi formulated his final defensive plan. A radical departure from the defensive tactics used by the Japanese earlier in the war, this plan provided for the following major points: in order to prevent disclosing their positions to the Americans, Japanese artillery was to remain silent during the expected pre-landing bombardment, and no fire would be directed against the US Navy vessels; when they landed, the Americans were not to encounter any opposition on the beaches but, after they had advanced some 550 yards (500 m) inland, they were to be taken under the concentrated fire of automatic weapons stationed in the vicinity of Motoyama Airfield No. 2 to the north, as well as automatic weapons and artillery emplaced both on the high ground north of the landing beaches and on Mt Suribachi to the south. After inflicting maximum possible casualties and damage on the landing force, the artillery was to move northward from the high ground near Chidori Airfield No. 1. In this connection, Kuribayashi stressed the conduct of an elastic defence designed to wear down the invasion force. Such prolonged resistance naturally required the defending force to stockpile rations and ammunition. To this end the island’s commander accumulated a food reserve to last for two and a half months, ever mindful of the fact that the trickle of supplies which was reaching Iwo Jima during the latter part of 1944 would cease altogether once the island was surrounded by the US Navy’s warships and transports. During the final months of preparing the defences, Kuribayashi saw to it that the building of fortifications did not interfere with the training of units. As an initial step toward obtaining more time for training, he ordered work on the northernmost of the island’s three airfields to be halted. In an operations order issued in early December, Kuribayashi set 11 February 1945 as the target date for completion of defensive preparations and specified that personnel were to spend 70% of their time in training and 30% in construction work.

Despite intermittent harassment by US submarines and aircraft, additional personnel continued to arrive on Iwo Jima until February 1945, by which time Kuribayashi had under his command a force of something between 21,000 and 23,000 men, including both army and navy units: a reasonable estimate suggests 13,585 army troops and 7,345 naval personnel. Kuribayashi made several changes in his basic defence plan in the months preceding the American invasion. The final version, which became effective in January 1945, called for the creation of strong, mutually supporting positions which were to be defended to the death without any thought of large-scale counterattacks, withdrawals or banzai charges. The southern portion of Iwo Jima, near Mt Suribachi, was organised into a semi-independent defence sector whose fortifications included casemated coast artillery and automatic weapons in mutually supporting pillboxes. The narrow isthmus north of Mt Suribachi was defended by only a small infantry force. On the other hand this entire area was exposed to the fire of artillery, rocket launchers and mortars emplaced on Mt Suribachi to the south and the high ground to the north. A primary defence line, consisting of mutually supporting positions in depth, extended from the north-western part of the island to the south-east, along a general line from the cliff north-west, across Motoyama Airfield No. 2 to Minami village, and thence eastward to the coast just south of Tachiiwa Point. This whole defence line was dotted with pillboxes, bunkers and blockhouses. Nishi’s immobilised tanks, carefully dug in and camouflaged, further strengthened this fortified area, whose defensive capability was enhanced by the broken and very rugged terrain. A second defence line extended from a few hundred yards south of Kitano Point at the very northern tip of Iwo Jima across the incomplete 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 further protection for the island’s two completed airfields against direct assault, the Japanese constructed a number of anti-tank ditches near the fields and mined all the natural lines of approach. When, on 2 January, more than a dozen Consolidated B-24 Liberator bombers raided Chidori Airfield No. 1 and inflicted heavy damage, Kuribayashi diverted more than 600 men, 11 trucks, and two bulldozers for immediate repairs. As a result, the airfield was rendered operable once more after only 12 hours. Eventually, 2,000 men were assigned the job of filling the bomb craters, as many as 50 men being detailed to each bomb crater.

The end of 1944 saw B-24 bombers over Iwo Jima almost every night, while US Navy carriers and cruisers frequently sortied into the Ogasawara Gunto and Kazan Retto. On 8 December 1944 US aircraft dropped more than 800 tons of bombs on Iwo Jima, a total which caused very little real damage to the island’s defences. Even though frequent air raids interfered with the Japanese defensive preparations and robbed the garrison of much badly needed sleep, progress on the defensive preparations was not materially slowed. On 13 February, a Japanese patrol aeroplane spotted 170 US ships moving north-west from Saipan. All Japanese forces in the Ogasawara Gunto and Kazan Retto were alerted and occupied their battle positions. On Iwo Jima, preparations for the pending battle had been completed, and the defenders were ready.

[Continued next week…]

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[First and third photos by John Anderson, courtesy of bunnygoth]