'Reckless' was the US seizure of the Hollandia area in the Japanese-held Netherlands New Guinea (22 April/6 June 1944).

In 1941 Hollandia was the largest settlement in the Dutch (western) half of New Guinea. The town was located on the only genuinely good natural harbour, Humboldt Bay, on the north coast of the Netherlands New Guinea, though it possessed only rudimentary port facilities: the two nearest ports were Wewak to the east and Geelvink Bay to the west. There was also a small airstrip. To the west, the Cyclops mountain range rises to heights of more than 7,000 ft (2135 m). To the south of the Cyclops mountain range and some 15 miles (24 km) long, Lake Sentani is about 6 miles (10 km) from Hollandia. A native track led from Lake Sentani to Tanahmerah Bay, 25 miles (40 km) to the west of Hollandia on the far side of the Cyclops mountain range.

Located on the eastern side of the large headland separating Humboldt Bay and Tanahmerah Bay, some 25 miles (40 km) to the west, Hollandia was occupied on 20 April 1942 by the Japanese, who then, about one year later, started to construct three airfields on the plain between the Cyclops mountain range and Lake Sentani and began construction of a fourth at Tami, 5 miles (8 km) to the east of Hollandia on the shore of Humboldt Bay.

In the spring of 1944 General Douglas MacArthur’s South-West Pacific Area command decided that the area should be seized for development as a staging post for the Allied forces' advance to the west along the north coast of New Guinea into the Netherlands East Indies and thence northward to the Philippine islands group. The port at Hollandia and the nearby airfields were currently the base area for units, totalling about 14,000 men, of General Korechika Anami’s 2^nd Area Army and Lieutenant General Giichi Itahana’s (from 1 April Major General Masazumi Inada’s) 6^th Air Division of the 4^th Air Army.

At a time early in 1944, the Allies decided to bypass Wewak, and jump straight to Hollandia, which was the best site short of Geelvink Bay for developing a major base. However, this would be the longest leap along the coast yet attempted by Vice Admiral Thomas C. Kinkaid’s US 7^th Fleet. Carrier support by elements of Admiral Chester W. Nimitz’s Pacific Fleet could be provided only for the first few days of the landings, and the nearest Allied airfield was at Nadzab, some 500 miles (805 km) distant. MacArthur therefore ordered the simultaneous 'Persecution' landing at Aitape, a lightly held Japanese position on the coast some 125 miles (200 km) to the east of Hollandia, in order to take the airstrip at Tadji and establish a blocking position for any movement to the west by the encircled Japanese forces at Wewak. 'Persecution' and 'Reckless' were scheduled for 22 April 1944.

The Japanese at this time intended to make the western part of New Guinea a major element of their inner defence perimeter, and in December 1943 Lieutenant General Fusataro Teshima’s 2^nd Army at Manokwari was assigned a new infantry division and Major General Shigeji Hakugin’s 7^th Air Division. In March 1944 the movement of two more divisions from China to the 2^nd Army was ordered. Lieutenant General Hatuzo Adachi’s battered 18^th Army in the eastern part of New Guinea was put under Teshima’s command at this time, and Teshima ordered Adachi to move his command to the west. Adachi ignored the order because he was convinced the Americans meant to land at Hansa Bay, between Madang and Wewak. The US air forces flew heavy bombing raids, ship bombardments, and patrols against Wewak in order to reinforce this impression. However, on 12 April Anami, who was senior to both Teshima and Adachi, sent Lieutenant General Takazo Numata, his chief-of-staff, to Wewak to demand compliance. As a result, two regiments of the 18^th Army were already on their way to the west by 22 April.

Taking the Allied forces directly from North-East New Guinea into the Netherlands New Guinea, this undertaking was therefore part of an ambitious three-pronged amphibious assault planned by the South-West Pacific Area command to outflank the forces of Adachi’s 18^th Army, which 'Ultra' intelligence had revealed to be holding the base areas of Madang and Hansa Bay in great strength and were still engaged in the east with the forces of Lieutenant General Edmund F. Herring’s Australian I Corps in the exits from the Huon peninsula.

'Reckless' was planned after the success of 'Brewer' in taking the Admiralty islands group, which prompted the US Joint Chiefs-of-Staff to issue a 12 March directive giving the South-West Pacific Area command a more prominent strategic role than had been envisaged until shortly before this time. 'Reckless' was in full accord with MacArthur’s 'Reno IV' plan, and it successful implementation would provide the US forces with the excellent anchorage in Humboldt Bay off Hollandia, and also with the airfield complex around Lake Sentani just inland of Hollandia, so speeding the advance of MacArthur’s forces toward the western end of New Guinea, where Geelvink Bay and the Vogelkop peninsula were needed as the base areas from which the US return to the Philippine islands could be launched.

The 'Persecution' and 'Reckless' operations were planned and executed in tandem to effect a strategic bypass of the major concentration of Japanese forces in the Wewak area, which would thus be isolated and left for the Australian forces to mop up. At a distance of 275 miles (445 km) to the north-west of Hansa Bay, 'Reckless' was a jump beyond the effective range of US land-based aircraft, and therefore depended on the US Navy’s provision of carrier task groups. Admiral Chester W. Nimitz and MacArthur met in Brisbane on 25 March, and after hard bargaining Nimitz was swayed by the arguments of Vice Admiral Thomas C. Kinkaid, commanding the South-West Pacific Area’s 7^th Fleet, and agreed that a force of his fleet carriers could remain off New Guinea for one day while a force of escort carriers could remain for seven days. In the event, Major General George C. Kenney’s 5^th AAF had effectively destroyed all Japanese air capability in the theatre by the time 'Reckless' was implemented by Lieutenant General Robert L. Eichelberger’s I Corps of Lieutenant General Walter C. Krueger’s 6^th Army.

Despite the fact that the Allied forces were advancing to the west along the coast of North-East New Guinea, late in 1943 the north central coast of New Guinea was essentially undefended in the region between Wewak in the east for some 370 miles (595 km) to Sarmi in the west. The Japanese now began to appreciate the need to reinforce the Hollandia and Aitape areas, which were nominally under the control of the 18^th Army some 200 miles (320 km) to the east. This hard-hit formation could not reinforce the Hollandia area, so Anami’s 2^nd Area Army, the formation controlling the 18^th Army, undertook the task itself. Lieutenant General Hidemitsu Nakano’s 51^st Division of the 18^th Army was ordered to move to the west in the direction of Hollandia, and Lieutenant General Jusei Aoki’s 20^th Division was instructed to garrison Aitape. Neither of these formations could complete its movement and thereupon assume its responsibilities until May. To prepare defences in advance, Major General Toyoza Kitazono’s 3^rd Field Transport Command was dispatched from Hansa Bay to Hollandia early in April. By this time the Hollandia area had been developed as the largest Japanese air base complex in this theatre outside the Rabaul area. There were at airfields at Hollandia, Sentani and Cyclops on the northern side of Lake Sentani, inland and about midway between Humboldt and Tanahmerah Bays. The headquarters of Vice Admiral Yoshikazu Endo’s 9^th Fleet and Lieutenant General Kumaichi Teramoto’s 4^th Air Army had both been located in this area, but had both been evacuated before the US landing.

At the time of 'Reckless', the Hollandia area was defended by some 14,000 army and navy troops, but only about one-fifth of these could be regarded as combat troops: these were one garrison and two anti-aircraft battalions, a few automatic weapons companies, and the 90^th Naval Base Force. The 3^rd Field Transport Command was the senior headquarters, and responsible for a mix of transport, engineer, airfield, air service, medical, signal and supply units as well as stranded aircrews. When they were alerted to the possibility of an Allied landing on 17 April, the Japanese rushed to improve their defences.

The final Allied developments Preparations for the 'Reckless' landings at Hollandia began between 30 March and 2 April, starting with a deep-penetration carrier raid against the Palau islands group, from which major elements of Admiral Mineichi Koga’s (from 1 April Admiral Soemu Toyoda’s) Combined Fleet posed a threat to any move against Hollandia. The US raid was successful in forcing the Combined Fleet to fall back to bases farther to the west. At the same time, the US 5^th Air Force attacked Hollandia with more than 80 Consolidated B-24 Liberator heavy bombers escorted by Lockheed P-38 Lightning heavy fighters modified for greater range. By mid-April Hollandia had been effectively destroyed as an air base complex, and the wrecks of more than 340 Japanese aircraft were later found on the runways, with an estimated additional 50 aircraft shot down over dense jungle. As a result of this debacle, the commander of 6^th Air Division was relieved in disgrace.

US intelligence suggested that there might be 14,000 Japanese at Hollandia, and probably 3,500 (but possibly 6,500) at Aitape. But the Japanese in New Guinea were in the throes of a reorganisation after their 'Postern', 'Michaelmas' and 'Dayton' defeats on the Huon peninsula, and as noted above the 18^th Army had recently come under the direct command of the 2^nd Area Army, which had ordered a withdrawal of Adachi’s three forward divisions from the area of Wewak to Hollandia, which was to become the main Japanese base in central New Guinea. At the time of the US assault there were in fact just some 12,500 Japanese in Hollandia, under Kitazono’s command, and 2,500 in Aitape. Most of these Japanese were administrative rather than combat troops, however, and at Hollandia there were in fact only 500 combat troops. Moreover, local commanders had also been reshuffled, and none of them had been in the area for more than a few weeks, Endo and Kitazono having arrived only three weeks and 10 days earlier respectively to lead the naval and army forces.

The 2^nd Area Army thought that Hollandia was a likely target, and had ordered the 18^th Army to reinforce the area with troops from Wewak and Hansa Bay, but Adachi believed that Wewak was a more likely target and had therefore stalled the departure of units to the north-west. Adachi was strengthened in his belief by the heavy US bombing of Wewak, which was part of a deliberate and highly successful deception.

Greatly aided by 'Ultra' intelligence, which provided an accurate assessment of Japanese strengths and dispositions, 'Persecution' was undertaken in parallel with 'Reckless' to take Aitape, 125 miles (200 km) to the east, where the airfield could support fighters used to cover 'Reckless'.

Over the same period, Vice Admiral Marc A. Mitscher’s Task Force 58, otherwise the Fast Carrier Task Force, was to support the Hollandia landing with a large-scale penetration of Japanese-held area. TF58 departed from Majuro on 13 April, was refuelled north of the Admiralty islands on 19 April and warplanes from its 12 carriers began their attacks on 21 April. Rear Admiral Joseph J. Clark’s Task Group 58.1, based on the fleet carrier Hornet and light carriers Belleau Wood, Cowpens and Bataan, launched fighter attacks on Wakde and Sarmi, and shelled them by night using the light cruisers Santa Fe, Mobile and Biloxi, and five destroyers. On 22 April there was another carrier raid, followed by other raids on 23 and 24 April. Rear Admiral Albert E. Montgomery’s TG58.2, based on the fleet carriers Bunker Hill and Yorktown, and light carriers Monterey and Cabot, carried out attacks on 21 April on Wakde and areas near Hollandia, and supported the landing in Humboldt Bay from 22 to 24 April. Rear Admiral Joseph M. Reeves’s TG58.3, based on the fleet carriers Enterprise and Lexington, and light carriers Princeton and Langley, attacked Hollandia on 21 April and supported the landings near Tanahmerah Bay from 22 to 24 April. There was very little resistance in the air after previous attacks by the 5^th Army Air Force had effectively destroyed Japanese air strength in the area. The US carrier force lost just 21 aircraft in these undertakings.

After refuelling on 27 April in an area to the north of the Admiralty islands group, TF58 carried out a major attack on Truk on 29/30 April. Of the 104 Japanese aircraft at Truk, 59 were destroyed in air combat and 34 on the ground, while the US carriers lost 26 aircraft shot down and nine in accidents.

On 29 April aircraft of the VT-30 torpedo bomber squadron from Monterey, in company with the destroyers MacDonough and Stephen Potter, sank the Japanese submarine I-174 to the north of Truk. The US submarine Tang rescued 22 downed aircrew, some of them within Truk lagoon.

On 30 April Rear Admiral Jesse B. Oldendorf’s Satawan Bombardment Group 1 shelled Japanese positions in the Satawan islands group to the south-east of Truk. On 1 May Vice Admiral Willis A. Lee’s Ponape Bombardment Group 2 shelled the island of Ponape with air escort provided by the aircraft of TG58.1. This group reached Eniwetok on 4 May, and TG58.2 and TG58.3 reached Majuro.

In 'Reckless', the I Corps was to land the 19^th and 21^st Regimental Combat Teams of Major General Frederick A. Irving’s 24^th Division in Tanahmerah Bay near Wakde in the west of the operation, and the 162^nd and 186^th Regimental Combat Teams of Major General Horace H. Fuller’s 41^st Division in Humboldt Bay in the centre of the operation. The 24^th Division was then to push to the east round the Cyclops mountain range and advance to meet the 41^st Division driving inland from Hollandia into the plain between the mountains and Lake Sentani, where the Japanese had built the airfield complex which was the operation’s primary objective.

Supported by Kenney’s 5^th AAF and Rear Admiral Daniel E. Barbey’s 7^th Amphibious Force (TF77) of the 7^th Fleet, the two invasion forces were backed by the 'Reckless' Task Force Reserve (34^th Infantry detached from the 24^th Division), and amphibious support was provided by the 2^nd Engineer Special Brigade.

The 24^th Division was the core of the 'Noiseless' Landing Force for Tanahmerah Bay, and in preparation for its assault had undertaken an incomplete rehearsal at Taupota Bay on the New Guinea coast to the south of Goodenough island, its base. The 24^th Division was to make the main effort to drive toward the operation’s primary objective, namely the airfields in the area of Lake Sentani.

The 41^st Division was the core of the 'Letterpress' Landing Force for Humboldt Bay: the division’s 163^rd Infantry had been detached for 'Persecution', which was to be launched against Aitape on the same day. Based at Finschhafen, the 41^st Division undertook its rehearsals near Lae, and was tasked with the implementation of a supporting attack from the east as the 24^th Division drove for the airfields from the west, and thus be in a position to trap any Japanese forces fleeing to the east from the 24^th Division’s assault.

Barbey’s TF77 was divided into a number of task groups for 'Reckless'. The three most important of these were Barbey’s own TG77.1 (Western Attack Group), Rear Admiral William M. Fechteler’s TG77.2 (Central Attack Group) and Captain Alfred Noble’s TG77.3 (Eastern Attack Group).

TG77.1 carried the headquarters of the I Corps, the 24^th Division and the 542^nd Engineer Boat and Shore Regiment with the destroyer Swenson, transport Henry T. Allen, Australian infantry landing ships Manoora and Kanimbla, dock landing ship Carter Hall, cargo ship Triangulum, 16 infantry landing craft and seven tank landing ships, all screened by the destroyers Hobby, Nicholson, Wilkes, Grayson, Gillespie and Kalk, and with six special service vessels in the form of the tug Reserve, two submarine-chasers, one infantry landing craft and two yard minesweepers.

TG77.2 carried the 41^st Division (less one regimental combat team) and the 532^nd Engineer Boat and Shore Regiment with the destroyer Reid, Australian infantry landing ship Westralia, dock landing ship Gunston Hall, cargo ship Ganymede, destroyers transports Humphreys, Brooks, Sands, Gilmer and Herbert, 16 infantry landing craft. and seven tank landing ships, all screened by the destroyers Stevenson, Stockton, Thorn, Roe, Welles, Radford and Taylor, and with nine special service vessels in the form of the high-speed minesweepers Hogan and Hovey, tug Sonoma, two submarine-chasers, two infantry landing craft and two yard minesweepers.

Under the command of Noble, who was Barbey’s chief-of-staff, TG77.3 carried Brigadier General Jens E. Doe’s 163^rd Regimental Combat Team toward Aitape with the destroyer La Vallette, destroyer transports Kilty, Ward, Crosby, Dickerson, Talbot, Schley, Kane, Dent and Noa, dock landing ship Belle Grove and cargo ship Etamin, all screened by the destroyers Nicholas, O’Bannon, Jenkins, Hopewell and Howorth, seven tank landing ships, and with nine special service vessels in the form of the high-speed minesweepers Hamilton and Perry, tug Chetco, four submarine-chasers and two yard minesweepers.

Back-up to these first-echelon forces was provided by two reinforcement groups and a floating reserve. TG77.4 was the First Reinforcement Group divided into three units: the Western Unit comprised the attack cargo ship Virgo, destroyers Stevens and Harrison, patrol frigate Coronado and six tank landing ships; the Central Unit with the destroyers McKee and John Rodgers, patrol frigate San Pedro and five tank landing ships; and the Eastern Unit with the cargo ship Bootes, destroyers Fletcher and Murray, patrol frigates Glendale and Long Beach, and six tank landing ships.

TG77.5 was the Second Reinforcement Group divided into two units: the Western Unit with the attack transports Zeilin and Windsor, destroyers Sigsbee and Dashiell, and destroyer escorts Lovelace and Manning; and the Central Unit with the destroyers Ringgold and Schroeder, and five tank landing ships. TG77.6 was the Floating Reserve with the attack transports Ormsby, Harry Lee and Centaurus.

Support for TF77 was provided by the escort carriers of Rear Admiral Van H. Ragsdale’s TF78, which was divided into two task groups 3. Gunfire support by heavier warships was entrusted to a pair of cruiser task forces 4.

Longer-range air reconnaissance was provided by TF73 or the Aircraft 7^th Fleet divided into TG73.1 (Seeadler Harbor Group) with the seaplane tender Tangier, small seaplane tenders Heron and San Pablo, and three squadrons in the form of VP-33 with 13 Consolidated PBY-5 Catalina flying boats, VP-52 with 13 PBY-5 'boats and VB-106 with 11 Consolidated PB4Y-1 Liberator patrol bombers; and TG73.2 (Langemak Bay Group) with the small seaplane tender Half Moon and one squadron in the form of VP-34 with 10 PBY-5 'boats.

The ships left the Goodenough island and Cape Cretin staging areas between 16 and 18 April, TG77.1 heading for Tanahmerah Bay and TG77.2 for Humboldt Bay. The convoys sailed round the eastern and northern sides of the Admiralty islands group and assembled to the north-west of Manus island on 20 April, along with the convoy bound for the 'Persecution' assault on Aitape, before approaching their target areas.

Dawn on 22 April was overcast, with light rain and rough seas, which was an impediment to the provision of tactical air support, but also served to conceal the approach of the Allied ships until the last minute as US and Australian cruisers and destroyers opened fire on the landing beaches.

The landing at Tanahmerah Bay almost became a disaster. Allied intelligence had incorrectly concluded that there was a partially completed road from here to Lake Sentani, a remnant from an abandoned Dutch colonisation project, and aerial photographs seemed to show two good landing beaches. However, an Australian ground scouting party sent to the area on 23 March had been betrayed by the local population, ambushed by the Japanese, and scattered into the jungle before it could assess and report in the terrain. Later aerial photographs suggested the beaches were not as ideal as originally thought, but by then the planning process had progressed so far that Krueger concluded it was too late to change the plan. When the landing force came ashore, it found that Red 2, the more northerly of the two selected beaches, was almost ideal for the beaching of landing ships and craft, but lay in front of a swamp which was completely impassable to men and vehicles. Thus there was no way even for infantrymen to get off the beach, which quickly became congested. The other beach, the more southerly Red 1, was only 100 yards (90 m) wide and located in a cove full of coral heads that prevented anything larger than an amphibious tractor or mechanised landing craft from reaching the beach: the terrain behind Red 1, however, was at least traversable if confused and steep. The process of shifting men and supplies by boat from Red 2 to Red 1 soon began, and a naval demolition party began blasting a clear channel for landing ships to approach Red 1 directly.

The 24^th Division’s 19^th Regimental Combat Team and part of the 21^st Regimental Combat Team came ashore in landing craft on the Red 2 beach, some 800 yards (730 m) long, on the north-eastern side of Tanahmerah Bay at 08.00. Only very light small arms fire was received, and there were only a few US casualties. The ground behind the beaches proved to be both hilly and swampy, making movement inland difficult. More critical, however, was the scarcity of suitable ground for supply dumps, vehicle parks, assembly areas, and bivouac sites, and as a result the beach-head soon became choked, as noted above. An accompanying tank company of the US Marine Corps had to be re-embarked as the terrain was so rough it could not park, let alone manoeuvre. Unloaded artillery sank into the mud.

One company of 1/21^st Regimental Combat Team landed unopposed in amphibious tractors on the narrow Red 1 beach inside Déparpré Bay, which was fronted by a previously unknown coral reef. The rest of the battalion came ashore in landing craft. Scheduled for 07.25, the landing was delayed 20 minutes as the amphibian tractors negotiated the reef. The move inland was further delayed by swamps, and the maze of wandering trails made it difficult to find the trail leading to Lake Sentani. This battalion was to advance toward Lake Sentani and the airfields as soon as possible with the rest of the division following from Red 2 beach via Red 1 beach. Patrols from Red 2 beach expected to find a road or trail linking the beach-head with Red 1 beach about 2 miles (3.2 km) to the south, but such a road did not exist.

The combination of a simple jungle trail and the ruggedness of the terrain, delaying the construction of a road, virtually immobilised the division and caused drastic changes in plans. Masses of troops, men and supplies were piling up in the beach-head and there was no way to move them over the intervening swampy and hilly terrain to Red 1 beach and the trail to Lake Sentani. Landing craft were used to shuttle men and supplies to Red 1 beach, but this also had insufficient area for dumps and assembly areas. Follow-on supply convoys bound for Tanahmerah Bay therefore had to be diverted to Humboldt Bay for lack of area in the US 24^th Division’s two very cramped beach-heads. Matters were exacerbated by the discovery that the 14-mile (22.5-km) 'road' to the airfields, previously thought capable of supporting vehicular traffic, was a crude native track with hairpin turns on the hillsides climbing 60° slopes and regularly swept by mud slides. Vehicle movement was out of the question, but the 1/21^st Regimental Combat Team managed to advance up the trail with the 3/21^st Regimental Combat Team following. At first no Japanese opposition was encountered, but the slow rate of supply delivery severely slowed the two battalions' advance.

Two infantry battalions, two anti-tank companies, two light artillery companies, and elements of various service and support units, totalling 3,500 men, were detailed to manpack supplies for the two forward battalions, but this proved to be an essentially impossible task as the rains increased and the distance to the forward elements extended. The weather cleared enough to allow limited supply drops on 26 April, and the units made it to Hollandia airfield after a huge effort. The Japanese resistance had been only light, but in combination with the terrain and climatic conditions served to delay the advance still further.

In spite of the need to manpack supplies up the trail, a task which diverted 3,500 combat troops, and in spite of skilful rearguard actions by small groups of Inada’s troops, the two regiments reached the westernmost airfield at Sentani on 26 April and established contact with units of the 41^st Division advancing from Humboldt Bay. The weather also improved enough to permit the air-dropping of badly needed supplies. It then took three months for engineers to complete a satisfactory road from Tanahmerah Bay to Sentani.

Meanwhile, at 07.00 on 22 April the 41^st Division landed at the south-western end of Humboldt Bay, where the greatest Japanese resistance was expected, with the 162^nd Regimental Combat Team landing on White 1 beach (a sandy but narrow beach surrounded by swamp whose only exit was dominated by a high plateau to the north, Pancake Hill, where the Japanese had emplaced a number of anti-aircraft guns) on the right flank and securing the village of Hollandia to the north after the Japanese positions on Pancake Hill had been flattened by a barrage of artillery rockets from two adapted infantry landing craft, and engineers had bulldozed a road past Pancake Hill. By the fall of night, some 4,200 tons of supplies had been unloaded and 300 vehicles were ashore.

The 162^nd Regimental Combat Team was followed ashore by the 3/186^th Regimental Combat Team, which landed on White 2 beach on the northern peninsula enclosing Jautefa Bay. This beach was on a narrow spit of land enclosing the small Jautefa Bay, where the Japanese had built a landing and a road to bring supplies in to their airfields. The landings on White 2 beach were made by DUKW amphibious trucks and LVT amphibious tractors which were to press forward across the spit and cross Jautefa Bay to seize the landing. Thus the assault troops were to have crossed the peninsula, driving through the mangrove swamps on its far side in their amphibious tractors, then plunged into the inner bay before coming ashore once again on White 4 beach on the north-western side of that bay. The amphibious tractors could not make it through the swamps, however, and so swam through the bay’s entrance between the two peninsulas to land on White 4 beach and secured the road from Hollandia inland toward the airfields. A single company of the 186^th Regimental Combat Team landed on White 3 beach, on the southern peninsula, to secure the southern flank.

Most units experienced only very light opposition. The 41^st Division had its own logistics problems, but these were not as serious as those of the 24^th Division. White 1 and White 2 beaches became very congested with supplies and equipment, a matter made worse by stacks of Japanese stores piled in the area as the Japanese had used Humboldt Bay as their own supply transfer point, White 3 beach was very steep, as were the other beaches, and on the wrong side of the bay to be of use.

On the morning of 23 April the 186^th Regimental Combat Team began its advance to the south before its planned swing to the west in the direction of Lake Sentani, 8 miles (13 km) from the bay. The road was much better than any other found in the area, but heavy rains delayed vehicle movement and Japanese resistance increased as the unit moved west. By the end of 23 April the 186^th Regimental Combat Team was half of the way to Sentani and the 162^nd Regimental Combat Team had taken Hollandia and the area surrounding it. Neither had encountered anything but minor resistance.

The landings suffered a serious setback during the night of 23/24 April 1944, when a single Japanese warplane dropped a string of bombs across White 1 beach. The stacks of supplies caught fire, and the area was soon devastated by exploding ammunition. Landing craft were used to evacuate troops from the beach, but 24 men were killed and 100 wounded, and more than 60% of the supplies landed on the beach were destroyed. The fires did not die down enough to allow engineers to return until 27 April, and for a time all supplies had to come over White 2 before being trans-shipped across Jautefa Bay by landing craft.

Even so, the main effort of 'Reckless' was shifted from the 24^th Division to the 41^st Division. A jetty on the eastern end of the lake was reached on 24 April, and was earmarked for use as a base for future operations. The 34^th Regimental Combat Team of the 24^th Division, the 'Reckless' Task Force Reserve, was landed in Humboldt Bay on 24 April after being diverted from Tanahmerah Bay.

As units pushed along the lake’s northern shore, two companies of the 1/186^th Regimental Combat Team, embarked in amphibious tractors at the jetty at 10.00 on 25 April, crossed the lake to make an unopposed landing at 11.50 up the lake close to the village of Nafaar on the north central shore, between Cyclops and Sentani airfields. A second such operation took place at 08.00 on 26 April, when two companies of the 2/186^th Regimental Combat Team left Nefaar to land at Ifaar 1 mile (1.6 km) farther to the west and near Cyclops airfield at 10.00 in the face of only light Japanese fire. The rest of the battalion landed there in the early afternoon. Only scattered resistance was encountered as the American forces approached the airfields, which had all been secured by the end of 26 April. At 16.45 patrols of the 186^th Regimental Combat Team, to the west of Hollandia airfield, made contact with the 21^st Regimental Combat Team moving slowly from the west.

The US supply situation was eased somewhat by the seizure of the airstrip at Tami on 1 May and bringing in transport aircraft from 3 May on to shuttle supplies from Humboldt Bay to the Sentani airfields.

Ironically, given the task for which 'Reckless' had been conceived, the Sentani airfield complex proved unable to support heavy bombers without extensive and lengthy engineering work.

Teshima wished to despatch two regiments from Wakde against the landings, but Anami authorised him to send only two infantry battalions and one artillery battalion on the long overland march. The three battalions were only about mid-way to Hollandia when US forces made their 'Straightline' landing at Wakde and isolated this Japanese force from its base, forcing them to turn back. Anami himself wanted to dispatch the 36^th Division from Sarmi to Hollandia, but this plan was vetoed by Field Marshal Count Hisaichi Terauchi’s Southern Expeditionary Army Group.

US mopping-up operations started to take lace in the area, which was declared secure on 6 June, even as the US development of this base area got under way. On 25 May the 41^st Division departed Humboldt Bay for the 'Horlicks' landing on Biak island. Most of the Japanese had withdrawn to the west, bypassing the 24^th Division to their south as they struggled east along its native track. The Japanese completed their assembly at the village of Genjem about 15 miles (24 km) to the west of Lake Sentani, where the Japanese had an agricultural project from which Inada hoped to supplement his meagre rations, and then headed for Sarmi, another 120 miles (195 km) or so to the west. Regardless of the general Japanese withdrawal, almost 1,000 Japanese stragglers in the area were killed during the US mopping-up operations.

Between 22 April and 6 June US losses in the Hollandia area had been 124 men killed, 28 missing and 1,057 wounded, while those of the Japanese had been more than 3,300 men killed and 611 taken prisoner.

Japanese elements retreating from Aitape in the east had assembled at the village of Arso about 20 miles (32 km) inland and south of Humboldt Bay by about 1 June. They then proceeded westward, but most were never again seen.

This major US success completely destroyed the integrity of the Japanese defence line fixed at the end of 1943. With Wakde and Sarmi now no longer feasible as main bastions, Imperial General Headquarters in Tokyo ordered the establishment of a new line running through Biak island and Manokwari, later modified to Sorong and Halmahera island with Biak island and Manokwari as forward outposts. The remnants of the 18^th Army were now faced with the problem of retreating some 400 miles (645 km) to join the 2^nd Area Army in western New Guinea. Though his orders instructed him to bypass the new US positions, Adachi was all too aware of the problems of such a retreat and decided to attempt at breakthrough at Aitape and Hollandia, an intention of which the Americans were first warned by local patrol successes and then more fully informed by 'Ultra' intelligence, allowing the Americans to strengthen their perimeter and establish strong defensive positions along the Driniumor river for Major General Charles P. Hall’s US XI Corps.

Although the 18^th Army's nominal strength was two infantry divisions, it had a mere 20,000 men of whom only 8,000 were combat troops. The Japanese attacked on 10 July, at one stage managed to penetrate the American defensive line, and suffered some 9,000 casualties before conceding defeat by 25 August. The battle for Aitape was less severe than that for Hollandia, and cost the Americans 87 dead to a Japanese equivalent of 4,441 before Adachi moved inland to face the dire prospect of a long westward infiltration without supplies and medicine.

Development of the newly taken Hollandia base area began quickly as the area was turned over to the Services of Supply on 6 June. The existing airfields were expanded by the USAAF, and Base 'G' was built round Lake Sentani, the largest in north-western New Guinea. In the original plan six airfields were to have been built at Hollandia, but because of swamplands and rain only the three existing fields were rebuilt. This massive complex housed the headquarters for the US Army in the Far East, Allied Land Forces, Allied Air Forces, 7^th Fleet, and 6^th and 8^th Armies as well as troop-staging camps. The Naval Advance Base, Hollandia, was built on Humboldt Bay, while unloading facilities, supply and ammunition depots, ship repair facilities, and support installations were built there. A fuel depot was built on Tanahmerah Bay. These bases were invaluable for staging and supporting operations in the southern part of the Philippine islands. The Allied headquarters were moved forward into the Philippine islands group as the offensive continued.