Operation Horlicks

'Horlicks' was the US seizure of Biak island, largest of the Shouten islands group in Geelvink Bay, off the north-west coast of Japanese-occupied Netherlands New Guinea by elements of Major General Horace H. Fuller’s 'Hurricane' Force (reinforced 41st Division) of Lieutenant General Robert L. Eichelberger’s I Corps in Lieutenant General Walter C. Krueger’s 6th Army within General Douglas MacArthur’s South-West Pacific Area command (27 May/30 August 1944).

Biak is the largest of the Schouten islands in Geelvink Bay on the northern coast of western New Guinea. The island is about 45 miles (72.5 km) long and 25 miles (40 km) wide. The somewhat smaller island of Soepiori is located to the north-west, across a very narrow strait, and the small islands of the Padaido group are scattered to the south-east. Biak itself is mostly a low plateau covered with jungle and reaching a maximum height of 2,428 ft (740 m) in the north-west, but on the island’s southern side there is a coastal plain with considerable areas of flat ground suitable for establishment of airfields. There are no natural anchorages and the island is almost completely surrounded by a formidable reef. The island’s administrative centre was Bosnik, on the south-east coast, where the reef was slightly narrower and crossed by two stone-built jetties. Biak’s native population was about 25,000 in 1944.

There is a 200-ft (60-m) escarpment extending from east of Bosnik to the village of Mokmer, and this escarpment is about 500 yards (460 m) inland along most of its length, but comes close to the shore at Parai, leaving just enough room for a road. Behind the escarpment, the terrain drops to a plateau, about 100 ft (30 m) in height, which is generally flat but in places comprises rough ground. The plateau was covered with scrub, transitioning to dense forest 2 to 3 miles (3.2 to 4.8 km) inland. To the west of the village of Mokmer, the escarpment turns to the north and extends inland for about 1.5 miles (2.4 km)before turning west again and running to the south-west corner of the island, and it is this which creates a relatively broad coastal plain suitable for airfield construction. The escarpment is coral with numerous caves, some of which are quite large.

The Japanese occupied the island in the spring of 1942 and established an airfield, on the coastal plain to the west of Mokmer, capable of handling bomber operations. The 222nd Regiment of Lieutenant General Hachiro Tagami’s 36th Division was redeployed from China in late 1943 to garrison the island. By 1944 there were additional airstrips at Bosnik and Sorido, and the entire airfield complex was protected by substantial concentrations of dual-purpose and anti-aircraft guns. The 19th Naval Guard Force was sent to the island in May 1944, just in time for the Battle of Biak, which was particularly bitter, costing the Americans and Japanese 524 and 5,093 dead respectively.

As noted above, on Biak the Japanese had built three airfields, one of which was still incomplete at the time of 'Horlicks', and planned another two airfields. The island was the responsibility of General Korechika Anami’s 2nd Area Army, and was defended by Colonel Naoyuki Kuzume’s 12,000-strong Biak Detachment, which was based on the 3,400 men of Kuzume’s own 222nd Regiment of Tagami’s 36th Division but also included one tank company with 10 Type 95 light tanks, the 17th, 107th and 108th Aircraft Construction Units each with 500 men, and Rear Admiral Sadatoshi Senda’s 28th Special Naval Base Force of 1,500 men of whom the only combat-trained element was the 19th Naval Guard Force 29th 125 men. With the addition of numbers of anti-aircraft gunners, artillerymen, engineers and service troops, Kuzume thus had 11,400 men, of whom about 4,000 were combat-trained.

The Japanese also planned to reinforce the Biak Detachment with the 221st Regiment of Lieutenant General Shunkichi Ikeda’s 35th Division from Halmahera island to the west.

The Americans wished to seize the island for the establishment of a heavy bomber base, for the Allies' only good heavy bomber bases in the South-West Pacific Area were at Nazdab on the Huon peninsula of New Guinea and in the Admiralty islands, and these were now too distant to the east to support MacArthur’s continued westward and northward movements in the direction of the Philippine islands group, and the advance of Admiral Chester W. Nimitz’s Pacific Ocean Areas in the direction of the Palau and Mariana island groups. The 'Reckless' operation against Hollandia had been designed to yield such bases, but it was then discovered that the earth in the area around Hollandia was inadequate to support heavy bombers without considerable construction work, which would have required many months and therefore delayed the progress of the US offensive, and the same applied to the airfield on Wakde island seized in 'Typhoon'. However, the airfield on Wakde did provide a forward base for the tactical aircraft which could be used to support 'Horlicks'.

Aerial reconnaissance had revealed that Biak was the nearest promising location for bomber airfields along MacArthur’s axis of advance and for support of Nimitz’s landings in the Palau and Mariana island groups. MacArthur therefore ordered Krueger’s 6th Army to plan and execute an expedited assault schedule on 10 May 1944, with the preliminary 'Typhoon' assault on Wakde to take place during 17 May so that airfields could be secured for the provision of local air cover, and an assault on Biak a mere 10 days later.

'Horlicks' had therefore to be planned in great haste, both to ensure the airfields would be in operation by the time of the 'Forager' assault on the Mariana islands group scheduled for 15 June and to ensure that momentum would not be lost for MacArthur’s drive toward the Philippine islands group. As commander of the land operation, Fuller was under considerable pressure to ensure that the airfields would be secured by 10 June.

Aerial reconnaissance showed that the reef off Mokmer was unfavourable for the passage of landing craft, while the coast farther to the east toward Parai was heavily fortified and the terrain between Parai and Bosnik was mostly mangrove swamp providing little room for manoeuvre. Thus it was decided that the 'Horlicks' landing would be made at Bosnik despite the fact that the landing force would have to advance a considerable distance along the coast road below the escarpment to reach the airfields at Mokmer.

Because conventional landing craft would not be able to cross the reef even at Bosnik, the plan called for six tank landing ships to launch 63 LVT tracked landing vehicles and 25 DUKW amphibious trucks to bring the first four assault waves ashore. Subsequent echelons would land at the jetties from 15 LCI infantry landing craft. Engineers and their equipment would be landed on the reef early in the invasion from eight LCT tank landing craft to prepare beaching ramps for LSTs carrying tanks.

The LVTs were manned by men of the 542nd Engineer Boat and Shore Regiment, while the DUKWs were manned by African-American troops. The DUKW drivers were hastily trained to operate out of LSTs at a rehearsal held at Hollandia on 23 May 1944, and performed well in the actual landing despite a considerable degree of confusion.

Preliminary air attacks by a total of 999 Consolidated B-24 Liberator heavy bombers were carried out by both Lieutenant General George C. Kenney’s (from 5 June Major General Ennis C. Whitehead’s) 5th Army Air Force and Major General Hubert R. Harmon’s 13th AAF from Nadzab and the the Admiralty islands group on 17 May 1944. The attacks continued on an almost daily basis until the landing date, and were joined in the last three days by the efforts of Douglas A-20 attack bombers based at Hollandia. Other attacks, by Liberator bombers working from Darwin in Australia’s Northern Territory, targeted the Japanese airfields in the Vogelkop peninsula to the west of Biak.

Kuzume’s garrison on Biak, reinforced by another 1,200 men during the fighting at the cost to the Japanese navy of two destroyers, was still completing the island’s defences as the US forces arrived, after a small-scale training landing in Humboldt Bay.

The US assault force was Rear Admiral Daniel E. Barbey’s Task Force 77 of Vice Admiral Thomas C. Kinkaid’s US 7th Fleet, and was centred on Rear Admiral William M. Fechteler’s Task Group 77.2 (7th Amphibious Force) with the destroyers Reid and Swenson, the high-speed transports Herbert, Kilty, Ward, Crosby and Schley, 15 LCIs and six LSTs. These were screened by the destroyers Hobby, Nicholson, Wilkes, Grayson, Gillespie, Kalk, Stockton, Roe, Welles, Sampson, Warrington and Balch. TG77.2 also included five submarine chasers and two LCIs as special service vessels. There was also Destroyer Squadron 2 with Morris, Anderson, Hughes, Mustin, Russell, Ellet, Lansdowne and Lardner.

On 25 May TG77.2 embarked Fuller’s 'Hurricane' Force, which comprised the 41st Division less its 163rd Regimental Combat Team, the 603rd Tank Company with 12 M4 Sherman medium tanks, two anti-aircraft battalions, two artillery battalions and three aviation engineer battalions, and departed for Biak during the evening of the same day.

The amphibious forces of TG77.2 were joined during the morning of the following day by the cruiser forces which provided cover. These were Rear Admiral V. A. C. Crutchley’s Australian TF74 (Covering Group 'A') with the heavy cruisers Australia and Shropshire, and destroyers Warramunga, Arunta, Ammen and Mullany; and Rear Admiral Russell S. Berkey’s US TF75 (Covering Force 'B') with the light cruisers Phoenix, Nashville and Boise, and destroyers Hutchins, Bache, Daly, Abner Read and Bush comprising Destroyer Squadron 24. Other naval elements were TF73 (Aircraft 7th Fleet) with TG73.1 (Seeadler Harbour Group) with the seaplane tender Tangier, small seaplane tenders Heron and San Pablo, the VP-33 squadron (13 Consolidated PBY-5 flying boats, VP-52 squadron (13 PBY-5 'boats) and VB-106 squadron (11 Consolidated PB4Y-1 Liberator patrol bombers) and TG73.2 (Langemak Bay Group) with the small seaplane tender Half Moon and the VP-34 squadron (10 PBY-5 'boats).

Land-based fighter cover was provided by Brigadier General Paul B. Wurtsmith’s V Fighter Command from Wakde.

Fechteler did not expect to achieve surprise, but the Japanese reconnaissance aircraft which spotted his force on the following day itself remained detected, and Fechteler therefore incorrectly believed he had achieved surprise after all.

The Japanese had been expecting an assault on Biak since the 'Reckless' assault on Hollandia, but did not believe that MacArthur’s forces would move as swiftly as they did to create and launch 'Horlicks'. Biak had originally been schemed as an important link in the Japanese defensive perimeter, but the Japanese high command had then wavered as it sought to decide whether Biak should be held or merely sacrificed to buy the time the Japanese needed to create a stronger perimeter farther to the west. On 9 May 1944 Imperial General Headquarters announced that the defensive perimeter was to be pulled back to Sorong, a port near the western tip of New Guinea, then reversed its thinking two days after the start of 'Horlicks' and ordered a powerful naval response.

Kuzume expected the US landing to take place near Mokmer and concentrated his forces accordingly, and then the assault of 178 May on Wakde prompted the Japanese commander to terminate work on the airfields in favour of extensive fortification work. Emphasis was put on the high ground to the north and north-west of Mokmer airfield in order to deny the Allies the use of this airfield for as long a time as possible. Fortifications included four 4.72-in (120-mm) dual-purpose guns and one 5.91-in (150 mm) coast-defence gun close to the beach. Other artillery, mortars, and automatic weapons were located on the escarpment behind Mokmer, with the strongest position in a set of limestone caves 1,200 yards (1100 m) to the north of Mokmer airstrip. The Allies later discovered that these 'West Caves' were centred on three sumps, the largest of which was 100 ft (30 m) wide and 80 ft (24 m) deep, and could accommodate 1,000 men. Another strongpoint was located in a smaller cave system, the 'East Caves', directly to the north of the village of Mokmer, to the east of the airstrip, while a third strongpoint with pillboxes was located just to the west of Parai and became known as the 'Ibdi Pocket'. Though based on what the terrain had to offer, the latter two strongpoints happened to be located in positions ideal for the ambushing of the US advance along the coast.

The warships of the Western Attack Group undertook a bombardment of the Japanese coast defences, which put up a stiff resistance, and on 28 May the destroyer Stockton was damaged by a Japanese shore battery off Biak. Elements of the 41st Division began to land at 06.29 on 27 May. The Japanese opposition at Bosnik was light except for a troublesome 4.7in (120-mm) gun situated in the Mokmer strongpoint: this damaged a destroyer and was not permanently silenced for several days. However, the weather was totally calm and the preliminary bombardment by the warships and the aircraft of the 13th AAF soon obscured the landing area with smoke and dust. Some 400 rounds of 8-in (203-mm), 1,000 rounds of 6-in (152-mm), 3,740 rounds of 5-in (127-mm) and 1,000 rounds of 4.7-in (120-mm) ammunition had been provided for the saturation of the area around the airfields, and another 4,900 rounds of 5-in (127-mm) and 4.7-in (120-mm) ammunition for the landing beaches at Bosnik.

The initial five waves of landing craft were caught in a 2- to 3-kt current, and landed about 2 miles (3.2 km) to the west of the planned landing beach before the destroyer Kalk located the stone jetties on radar and discovered the mistake. The first assault wave found itself in a mangrove swamp but rapidly regrouped, and the sixth and subsequent waves were directed to the correct beach. The jetties were found to be in good condition, and the remainder of the landing went according to plan.

By the fall of dark on 27 May, about 12,000 troops, 12 tanks, 18 pieces of artillery, 500 vehicles and 1,400 tons of supplies had been landed. The 186th Regimental Combat Team was able to advance from the swamp to the coastal road, and the 162nd Regimental Combat Team was ashore on the correct beaches.

The initial confusion during the landings led to the first controversy of the battle. The commander of 186th RCT, finding himself on the coast well to the west of Bosnik, recommended that his regiment exchange missions with the 162nd RCT so that the latter would secure Bosnik while the former made all possible speed to the west to reach the target airfield. Fuller rejected this recommendation on the grounds that this might well compound the existing confusion. The 186th RCT moved east to secure the beach-head while the 162nd RCT drove to the west through the 186th RCT’s lines and began a rapid advance along the coast road towards Mokmer, reaching Parai before digging in for the night.

Only small numbers of Japanese warplanes appeared during this first day of 'Horlicks'. Four fighters appeared over the airstrips at 11.00 but quickly retreated. Two fighters and four bombers appeared late in the afternoon and succeeded in hitting an LST with bombs which failed to detonate. Three of the bombers were shot down, but one of these succeeded in crashing into SC-699, which caught fire and lost two men dead and eight wounded before the fires were put out and the submarine-chaser salvaged.

Opposition was so light the first day that Barbey speculated that the Japanese had in fact evacuated the island. This was far from the case. Although the Japanese had spotted the invasion convoy, they were caught out of position and relatively unprepared for the attack. The 2/222nd Regiment was in place in the East Caves, but the other units were mostly scattered across the inland plateau. The command situation was further confused by the presence of Lieutenant General Takazo Numata, the 2nd Area Army's chief-of-staff, and Senda, both on an inspection trip from the area headquarters at Manokwari. It appears that Numata ordered Kuzume to abandon his plans for a defence in depth and instead make counterattacks in accordance with the traditional Japanese emphasis on the offensive. Numata left the island on 15 June, after which Kuzume reverted to his concept of defence in depth. Senda remained on the island and shared the fate of the garrison, placing himself under Kuzume’s tactical command.

MacArthur had discounted intelligence, which was soon shown to be accurate, indicating that the island was held by 11,000 Japanese troops and a number of light tanks, instead preferring to believe that there were no more than 4,400 defenders. The intelligence assessment was confirmed when the 162nd RCT began to close on the airfield on the morning of 28 May. The forward elements had closed to points within 200 yards (185 m) of Mokmer airfield before the Japanese suddenly opened fire on them from the East Caves and the Ibdi Pocket, taking the Americans completely by surprise. Elements of the 2/222nd Regiment attacked from the west and north and soon had the 3/162nd RCT trapped in a perimeter 500 yards (545 m) long and 200 yards (185 m) deep. The 1/162nd RCT tried to outflank the Japanese by advancing along the ridge from Parai, but was checked by fire from the East Caves. A platoon of Sherman tanks helped drive back an attack supported by Japanese tanks, but the 1/162nd and 2/162nd RCT were unable to break through to relieve the 3/162nd RCT, whose casualties had to be evacuated and supplies delivered by landing craft. The naval support officer with the 162nd RCT was killed at about 12.00, and this was a severe hindrance to effective co-operation with the destroyers offshore. At 16.00 Fuller ordered the 3/162nd RCT to pull back behind the line of the 2/162nd RCT, a move completed by 19.00 with the aid of covering fire from four Sherman tanks.

Destroyer gunfire was unsuccessful in its efforts to drive back the Japanese during the night, and the Japanese moved the 1/222nd Regiment to the West Caves from its bivouacs to the north and prepared to commit both the 2/222nd Regiment and 3/222nd Regiment against the Americans during the following morning. This counterattack was supported by light tanks, but these were completely outclassed by the Sherman tanks, which knocked out seven of the Type 95 light vehicles, and the Japanese infantry also suffered heavy casualties. By 12.00, however, the Japanese had cut the coastal road at Parai and it had become clear that the 162nd RCT’s position was untenable. The US were able to cover the evacuation of the 2/162nd RCT in LVTs while the rest of the 162nd RCT cleared the roadblock at Parai and fell back to Ibdi.

Fuller requested reinforcements for an attack to clear the escarpment, and was allocated two battalions of the detached 163rd RCT from Wakde, and the 503rd Parachute Infantry began to move to Hollandia in case it too was required. During the two days it took to bring up the two infantry battalions in LCIs, the 162nd RCT regrouped and rested, except for patrol activity that identified the Ibdi Pocket and found two trails across the escarpment. The Japanese also took the opportunity in this lull to regroup. Although some 800 men of the 3/222nd Regiment remained inactive in the Ibdi Pocket, Kuzume moved naval troops and a mortar company into the East Caves, reorganised most of the 2/222nd Regiment around the West Caves, and pulled the 1/222nd Regiment back to a position well to the north of Bosnik.

The 41st Division resumed its assault on 1 June: the 186th RCT advanced to the along the inland plateau while the 162nd RCT resumed its drive along the coast. The 1/222nd Regiment tried to counterattack during the night against the northern flank of the 186th RCT, but this effort was beaten back with heavy losses, with the Japanese battalion commander among the dead. The Japanese survivors continued withdrawing before the 186th RCT’s advance, and ultimately ended at the West Caves. After this the 186th RCT was hindered more by the terrain and lack of fresh water than by the Japanese: at one point, indeed, a tropical rainstorm was all that prevented the advance from halting for lack of water. Meanwhile the 162nd RCT had now appreciated that the Ibdi Pocket was a major strongpoint. After a pause on 3 and 4 June pending the outcome of 'Kon' (i), the US advance resumed.

By 6 June the 186th RCT was preparing to advance along the terrace to the north of Mokmer airfield in order to clear any Japanese positions overlooking the airfield. However, under pressure from Krueger to get at least one airfield operational as soon as possible, Fuller ordered the regiment to ready itself to move south immediately and seize the airfield and a beach-head round it. Because of inadequate patrolling by one of his battalions, the regimental commander failed to appreciate the fact that his unit was well positioned to outflank strong Japanese positions on the ridge to the north of Mokmer airfield. The 186th RCT instead headed directly to the airfield, under cover of a concentrated artillery barrage, and after occupying the airfield found itself under fire from the formidable Japanese positions on the ridge to the north. There followed an artillery duel in which some 40% of the Japanese guns were silenced, but the remainder kept the airfield from being put into operation. Moreover, the 186th RCT had been compelled to abandon its tenuous supply line back across the ridge and the inland plateau to Bosnik, and the Japanese at the Ibdi Pocket and the Parai defile had prevented the 162nd RCT from clearing the coastal road. Additional supplies had to be delivered by landing craft braving Japanese fire, which was possible only after infantry with tank support had cleared the Japanese fortifications along the beach to the south of Mokmer airfield.

The fight for Biak now became a battle of attrition rather than manoeuvre. Kuzume had the 1/222nd Regiment, 2/222nd Regiment, 19th Naval Guard Force and most of his army service troops and heavy weapons at the West Caves and the ridge to the north of Mokmer, some naval troops and mortars at the East Caves, and the 3/222nd Regiment in the Ibdi Pocket.

Fuller ordered most of the 3/162nd RCT to be moved by landing craft to the Parai jetty, from which it was to move east and, presumably, take the Japanese at the Parai defile from the rear. Instead, the battalion was hit by fire from the East Caves and called for armour and artillery reinforcements. Frustrated by the Japanese resistance at the Ibdi Pocket, Fuller decided to leave a single company to contain the pocket while the 162nd RCT cleared the Parai defile, opened the coastal road, and moved to the west to effect a junction with the 186th RCT at Mokmer airfield. The Ibdi Pocket was to be reduced by the 163rd RCT once this had arrived.

On 11 June Fuller launched a major attack by the 162nd and 186th RCTs to the west from Mokmer airfield. Each regiment put two battalions into the attack and held the third back at Mokmer airfield as a reserve. Meanwhile the 3/163rd RCT patrolled along the ridge to the north and north-east. Three of the assault battalions advanced against very light opposition, but the fourth and most northerly battalion, the 3/162nd RCT, almost immediately encountered determined resistance along the terrace. The battalion had stumbled onto the West Caves, and liberated Javanese labourers were able to confirm the presence of the Japanese stronghold. Company L of the 163rd RCT had meanwhile established an observation post on Hill 320, directly to the north of West Caves, and reported a major concentration of Japanese artillery around the West Caves. Concerned by the resistance his unit was facing, Fuller requested and received permission to use the 34th RCT of Major General Frederick A. Irving’s 24th Division.

On 12 June engineers began work on Mokmer airfield, and succeeded in repairing 2,300 ft (700 m) of runway by the evening of 13 June despite the harassment of Japanese mortar and artillery fire. Because of the delay in taking the airfields on Biak, meanwhile, army engineers explored the nearby Padaido islands group on 1 June, concentrating on Owi, Aoeki, and Mios Woendi islands off the south-eastern tip of Biak, and of these the first was selected for an airfield whose construction began on 9 June, though the field was not ready for use by fighters until 17 June. A PT-boat base was established on tiny Mios Woendi, 5 miles (8 km) to the south-east of Owi.

On 14 June the two reserve battalions were ordered to attempt to outflank the Japanese positions to the north. One of the battalions drove almost to the periphery of the West Caves, and Kuzume ordered a counterattack on the morning of 15 June, but this was checked and then pushed back by bazooka teams and artillery. Even so, the Japanese counterattack had spoiled the Americans' own plans for an attack on the same day.

MacArthur’s patience had been exhausted by this time, and on 15 June Fuller was succeeded in command by Eichelberger, whom Krueger instructed to take command of 'Hurricane' Force but also to leave Fuller in command of the 41st Division. Fuller felt he had lost the confidence of MacArthur and Krueger, however, and requested that he be replaced as divisional commander and reassignment outside the South-West Pacific Area. Thus Brigadier General Jens A. Doe, the assistant division commander, took command of 41st Division with a promotion to major general. Eichelberger renewed the US offensive on 16 June, and by the evening of 17 June two battalions had taken the high ground overlooking the West Caves. After a pause to regroup, most of the 162nd and 186th RCTs was committed against the West Caves, which had been isolated by the fall of night on 19 June. However, this strongpoint was not reduced until 27 June after the Americans had employed drums of petrol, hand grenades, direct fire from tanks and demolition charges to destroy the stubborn Japanese defence. A large force of Japanese made an unsuccessful attempt to break out on the night of 21/22 June. The fate of Kuzume in this attempt is uncertain: some reports claimed he committed ceremonial suicide immediately after the break-out attempt, and others that he was killed in action or committed suicide some days later. However, as the Americans closed the ring on the West Caves, engineering teams were finally able to work unhindered on Mokmer airfield, which could be used by fighters from 22 June.

Meanwhile Borokoe and Sorido airfields had been occupied by the 34th RCT, which had reached Biak on 18 June. The 34tth RCT was involved only briefly in the battle, and was returned to 6th Army reserve on 30 June, by which time the entire area to the north of the airfields had been cleared of the Japanese. There was brief but fierce resistance around a cul-de-sac to the north-west of Hill 320, dubbed 'The Teardrop', but this had been cleared by 25 June.

During this period the Americans had renewed their assault on the strong points at East Caves and the Ibdi Pocket. The East Caves position was held by about 1,000 men, most of them engineers and service troops bolstered by small numbers of mortar crews and infantrymen of the 222nd Regiment. From this position, the Japanese had fired intermittently on the airfields since 7 June, and US patrols had identified some of the Japanese positions. The 4at Division considered it more urgent to reduce the West Caves, however, and had therefore left the East Caves to be neutralised by artillery and mortars, which between 7 and 10 June dropped more than 1,000 shells onto the position. By 13 June the US artillery and patrols had suppressed the Japanese fire from the East Caves well enough that the coast road could at last be opened to truck traffic. US harassing fire continued until 27 June, and included a number of skip-bombing sorties by North American B-25 Mitchell medium bombers from Mokmer airfield on 23 or 24 June.

On 27 June the Japanese fire from the East Caves was troublesome enough to US engineers constructing a jetty near the village of Mokmer that mortar and artillery fire was again used to suppress the Japanese guns. Some 1,600 rounds were dropped into the caves, and the Americans increased their patrols into the area. On 3 July, engineers and infantry from Mokmer airfield moved into the East Caves under cover of tank fire and began blasting the entrances of the caves. The Americans broke into the main positions on 5 July to find that most of the troops had already slipped away. However, a few survivors remained in the area until 20 July, when the Americans responded to an ambush of Australian souvenir seekers by sending in tanks and infantry to mop up the position.

The attack on the Ibdi Pocket had begun on 1 June and initially made only slow progress. The Parai defile was finally cleared on 12 June, and the newly arrived 163rd RCT assumed responsibility for clearing the rest of the Ibdi Pocket while the 162nd RCT moved to the west to join the 186th RCT at Mokmer airfield. Between 12 and 20 June, the 163rd RCT scouted the escarpment, and then launched a major attack. This met resistance of a very stubborn nature, and by 28 June the exhausted US infantrymen were back to patrolling for the locations of targets to be tackled by artillery fire and air bombardment. From 21 June to 10 July, at least 40,000 artillery and mortar shells were fired on targets in the Ibdi Pocket, and on 10 July a patrol reported that the Japanese defenders were considerably reduced and that the heavy bombardment had stripped much of the cover from the Japanese positions. On the following day the ground assault was resumed by infantry with bazookas and flamethrowers, aided by armour, artillery fire and air support, and on 22 July all organised resistance had ceased. Mopping up continued until 25 July.

On 28 June the 2nd Area Army had ordered the remnants of the Biak Detachment to withdraw into the wilds of the island and prepare to wage a campaign of guerrilla warfare. Some 4,000 Japanese troops were still at large on the island, and most of these started to make their way to Wardo Bay on the west coast. The 41st Division now began to locate and destroy these remnants before they could reorganise. Patrols were sent far to the north of the escarpment, and on 2 August the 2/162nd RCT landed at Korim Bay, on Biak’s north-east coast, and moved to the south to make contact with the 3/163rd RCT on 15 August. The 1/186th RCT landed at Wardo Bay on 17 August, and three days later Biak was declared secure.

The Biak campaign had cost the Americans 474 men dead or missing and 2,428 men wounded. An epidemic of scrub typhus contributed another 1,000 casualties, and a further 3,500 men were diagnosed with tropical 'fevers of undetermined origin'. The Japanese lost about 6,100 known dead, many other wounded and about 450 taken prisoner, though some of the last were slave labourers.

It it also worth noting that the high command of the Imperial Japanese navy reacted strongly to the the US invasion of Biak, for this threatened to disrupt the 'A' plan for a decisive naval battle in the central Pacific. Vice Admiral Jisaburo Ozawa’s 1st Mobile Fleet, the main striking power of the Japanese navy, had already assembled at Tawi-Tawi, at the extreme south-western end of the Philippine islands group, in anticipation of this battle. The Japanese army had written off Biak, and the Japanese navy had allocated only the 18 aircraft of Rear Admiral Yoshiaki Ito’s 23rd Air Flotilla at Sorong to repel any attack on Biak.

The Japanese navy stated to concentrated its land-based air power in western New Guinea on 28 May 1944, ordering an additional 70 fighters and 20 bombers from Japan and the Mariana islands group to join the 23rd Air Flotilla. Another 20 bomber, 48 fighter and eight reconnaissance aircraft were redeployed from the Caroline islands group on 31 May. By then orders had been prepared, in considerable haste, for 'Kon' (i) to relieve Biak.

The Japanese assembled a strong force around the battleship Fuso, three heavy cruisers, one light cruiser, eight destroyers and two minesweepers to transport the 2,500 men of Major General Yoshio Tamada’s 2nd Amphibious Brigade from Zamboanga in the Philippine islands group to Biak. As only one dedicated transport, an LST, was available for the undertaking, most the troops were embarked on the warships on 31 May and expected to land on Biak during 3 June. Air cover was to be provided by Admiral Shiro Takasu’s South-West Area Fleet, which was also to arrange the delivery of additional troops by barge from Manokwari on the Vogelkop peninsula.

'Kon' (i) began with a major Japanese air raid against a group of eight LSTs unloading at Bosnik on the afternoon of 2 June. The Japanese mustered 54 aircraft for the raid and encountered no Allied fighter opposition as a result of adverse weather over Wakde and Hollandia. The anti-aircraft fire from ships and the shore was extremely heavy, though, and the Japanese lost 12 aircraft while inflicting only slight damage on one LST.

During the night of the same day the Japanese surface force, divided into three groups, set out for Biak. By 12.00 on 3 June the ships had been sighted, as a result of 'Ultra' decrypts of Japanese signal traffic, and were being shadowed by Liberator aircraft. The loss of surprise combined with mistaken intelligence reporting a US aircraft carrier off Biak persuaded Admiral Soemu Toyoda, commander of the Combined fleet to suspend the operation. Fuso, two heavy cruiser, and two destroyers returned to Davao in the Philippine islands group, but the destroyer Kazagumo was torpedoed and sunk by the submarine Hake as he approached the harbour. The remaining ships steamed to Sorong, disembarking their troops on 4 June.

The Japanese made a second air raid against Biak on 3 June, when three destroyers, eight LCTs and four LCIs off Bosnik were attacked by 32 Mitsubishi A6M Reisen 'Zero' fighters and 19 bombers. The attackers were detected on radar at 11.00 and the ships were able to avoid serious damage by violent manoeuvring. Initially delayed by the weather, Allied fighters arrived at 11.25 and drove off the remaining Japanese aircraft. The Japanese lost 11 aircraft but were rapidly reinforced by 70 aircraft flown from the Mariana islands group.

By this time Kinkaid had concluded that the Japanese meant to make a major naval effort to relieve Biak. Crutchley was ordered to refuel, then take Berkey’s cruiser group under his command, and this combined heavy and light cruiser force had reached a point 25 miles (40 km) to the north of eastern Biak by 19.00 on 4 June 1944. All other Allied forces were ordered to clear the area. The Japanese had sighted the Allied cruiser force with its escorting destroyers, and launched a raid which arrived at 17.40. One light cruiser was slightly damaged.

Crutchley had intelligence indicating that the Japanese meant to land on the west coast of Biak, and opted to sweep past the island’s south and west coasts, barely avoiding a 'friendly fire' incident when he passed the US shore batteries at Bosnik. No Japanese surface force was encountered and Crutchley retraced his course back to a patrol station to the north-east of Biak. Here the Allied cruisers were attacked during the night by four Japanese torpedo-bombers, which inflicted no damage. However, the Japanese launched a second night raid by two bombers against Wakde that caught more than 100 Allied aircraft parked closely together on the runway. This raid inflicted very heavy damage and put the airfield out of action for several days. So embarrassing was the episode that the US official history barely mentions the raid and quotes no casualties, but at least two-thirds of the aircraft on the island were put out of action.

The Japanese made a second attempt to deliver reinforcements in three barges towed by six destroyers. Two cruisers remained off the western tip of New Guinea to await developments while the 23rd Air Flotilla was under orders to provide fighter cover. The reinforcement flotilla embarked 600 troops at Sorong and departed for Biak at 24.00 on 7 June. The little flotilla was sighted by Allied aircraft at 12.45 on the next day, however, and a low-level strafing attack by 10 B-25 bombers sank the destroyer Harusame and inflicted slight damage on three other destroyers. Rear Admiral Naomasu Sakonju, the commander of the 16th Cruiser Division , pressed on with the five surviving destroyers, but at 19.00 was informed that an Allied task force was racing to intercept him.

This was once again the result of 'Ultra' decrypts, but Crutchley’s cruiser force had been delayed by manoeuvres to evade Japanese snoopers, and did not arrive off the west coast of Biak until 22.00 on 8 June. Soon after this a PB4Y spotted Sakonju’s force 60 miles (100 km) to the north-north-west. Crutchley ordered a turn to the north in order to cross the Japanese 'T', but Destroyer Division 42 ignored the order and made for the Japanese at maximum speed. The two forces sighted each other on radar at about 23.20, when Sakonju fired torpedoes and fled. Destroyer Division 42’s radioed a warning to Crutchley, which was confirmed when one of the torpedoes passed close astern of Boise. By then Crutchley’s remaining destroyers were also pursuing the Japanese destroyers. The Japanese scored no torpedo hits on the Allied ships, which were unable to catch the fleeing Japanese force, although the two forces came close enough to exchange gunfire and for the Japanese to launch more salvoes of torpedoes. The Allies finally gave up the pursuit at 02.30, lest they be mistaken for Japanese ships by their own aircraft when daylight came.

The Japanese had cast off the three landing barges early in the action. They were engaged with gunfire by the US destroyers as they passed them, but it seems that the barges made it to shore and landed a few reinforcements on Biak. However, the majority of the 600 troops in the reinforcement group were taken back to Sorong aboard the Japanese destroyers.

Finally, by 11 June 1944 Admiral Soemu Toyoda had detached from Vice Admiral Jisaburo Ozawa’s 1st Mobile Fleet an Attack Division (principally the super-battleships Musashi and Yamato under Vice Admiral Matome Ugaki) to destroy MacArthur’s right flank on the sea. The Attack Division had steamed scarcely more than half way to its starting point near Halmahera in the Molucca islands group before it was recalled for 'A' and the Battle of the Philippine Sea. This overwhelmingly powerful force had been scheduled to reach Biak on 15 June, deliver a heavy bombardment, and land troops, but the undertaking was cancelled on 12 June when heavy US air attacks on the Mariana islands group indicated that Admiral Raymond A. Spruance’s 5th fleet would soon begin its 'Forager' landings on the islands, which would trigger the Japanese 'A'. It has been suggested that the Japanese would have done better to stay with their 'Kon' (i) plan, which would probably have inflicted a serious defeat on the weak Allied naval forces off Biak, rather than send the super battleships to the north and the Battle of the Philippine Sea to which they contributed little. As it was, the 23rd Air Flotilla got in a final flourish before being withdrawn to Palau, inflicting serious damage on Kalk in a raid by four aircraft on the morning of 12 June.

The failure of 'Kon' (i) was costly for the Japanese. In addition to their naval losses, the diversion of so much of their land-based air power to western New Guinea seriously disrupted Japanese plans for 'A' and contributed to the debacle of the Battle of the Philippine Sea. Most of the pilots diverted to provide air cover for 'Kon' (i) promptly came down with malaria or other tropical diseases and as a result were unable to participate in either 'Kon' (i) or 'A'.

The Japanese subsequently managed to slip perhaps 1,100 troops in barges from Noemfoor to Biak through the Allied naval blockade. Most of these men were of the 219th, 221st and 222nd Regiments, generally landed at Korim Bay and then moved overland to the West Caves strongpoint.

During the battle, a Japanese naval radio unit ran into a US patrol and lost several code books. This prompted the Japanese to make emergency changes in their radio procedures, which slowed their communications just as the Battle of the Philippine Sea was getting under way.

Engineers had begun improving the facilities at Bosnik on the first day of the landing. On 30 May a naval demolition team blasted through the reef a channel, 5 ft (1.5 m) deep, to allow LCM mechanised landing craft and LCVP landing craft vehicle/personnel to land directly onto the beach. By 17 June an airstrip had been completed on Owi and two squadrons of Lockheed P-38 Lighting heavy fighters of Lieutenant Colonel Emmet S. Davis’s 8th Fighter Group arrived on 21 June. On 22 June the first fighters were operating from Mokmer airstrip. By 12 July the Owi airfield had a 7,000-ft (2130-m) runway, and a second was completed on 20 August. The airfield eventually had 20,000 ft (6095 m) of taxiways and 130 hardstands.

Mokmer airfield became operational on 22 June and by 1 August its runway had been extended to 7,000 ft (2130 m), with 27,000 ft (8230 m) of taxiways and 122 hardstands. A 4,000-ft (1220-m) runway had been completed for transport aircraft at Borokoe by 1 August and this had been lengthened to 5,500 ft (1675 m) by 20 August. Sorido was on less suitable ground, and while a 4,000-ft (1220-m) runway was briefly in use, the airfield was quickly abandoned. Unfortunately, none of these airfields was ready in time to give significant support to 'Forager', as had originally been planned.

Biak’s south coast had eight LST ramps, two floating docks for 'Liberty' ships, and five jetties at Bosnik, Parai and Mokmer by 20 August. The area thus became a major base for the Allies in the South-West Pacific Area, with depots and storage areas built at Base 'H' around Sorido to the west of the airfields. The US Navy built a seaplane base at Mios Woendi island, along with a landing craft repair facility, and in September a temporary advance submarine base. Other US Navy support facilities were built on nearby Noesi and Oreiv islands.

The 41st Division remained on Biak until the end of January 1945, and the Naval Advance Base, Biak, was closed on 19 January 1946.

The loss of Biak had been the first dent made in the Japanese perimeter fixed in autumn 1943 by General Hideki Tojo, the Japanese prime minister and minister of war, as being Japan’s defence line 'with no thought of withdrawal' and running from Timor island to the Mariana islands group via the north-western part of New Guinea and the Palau islands group.