The 'Battle of Driniumor River', sometimes known as the 'Battle of Aitape', was fought between US and Japanese forces in the area of Aitape in North-East New Guinea (10 July/25 August 1944).

The undertaking was part of the Western New Guinea campaign, and in the course of the fighting, Japanese forces launched several attacks on US forces on the Driniumor river near Aitape over several weeks with the intention of retaking Aitape. After making some initial gains, the Japanese attack was contained and eventually turned back having suffered heavy casualties. The battle is not to be confused with 'Persecution', which included US amphibious landings near Aitape in April 1944, or the Aitape-Wewak campaign, which began in November of the same year.

The Driniumor river lies some 20 miles (32 km) to the east of Aitape on the northern coast of what was then the Territory of New Guinea. During 1942 the Japanese had occupied much of New Guinea, but throughout 1943 the Allies had slowly gained a numerical and technical superiority. By a time early in 1944, the Allies had begun a series of landings along the northern and western coasts of New Guinea as part of the advance toward the Philippine islands group planned by General Douglas MacArthur. On 22 April, Allied forces landed at several key points around Hollandia and seized Aitape as part of 'Reckless' and 'Persecution', in the process cutting off Lieutenant General Hatazo Adachi’s Japanese 18^th Army, which was retreating to the west in the direction of General Korechika Anami’s Japanese 2^nd Area Army in the Netherlands New Guinea, and in the process bypassed strong Japanese positions around Wewak and Hansa Bay. After the seizure of Aitape, Brigadier General Jens A. Doe’s US 163^rd Regimental Combat Team consolidated its positions until a time early in May when it was relieved by Major General William H. Gill’s US 32^nd Division. A defensive perimeter was established around the area’s airfields, eventually extending 30 miles (48 km) to the east of Aitape and including several outposts along the Dandriwad river. From these positions, US troops patrolled to locate the Japanese troops in the area, which were centred on Wewak, about 90 miles (140 km) to the south-east of Aitape.

The Japanese forces holding the area were elements of the 18^th Army, which had sustained heavy losses in the Lae, Huon peninsula and Finisterre range campaigns and had received neither replacements nor reinforcements. Under the command of Adachi, the force now comprised about 20,000 men, and it primary combat formations were Lieutenant General Masutaro Nakai’s 20^th Division, reinforced with the 66^th Regiment from Lieutenant General Hidemitsu Nakano’s 51^st Division, and Lieutenant General Heisuke Abe’s 41^st Division. Intelligence derived from codebreaking as well as captured documents, Allied Intelligence Bureau patrols and other sources indicated that the 18^th Army was approaching the Driniumor river with the intention of breaking through the Allied line and retaking Aitape. In the period leading to the 'Battle of the Driniumor River', the Japanese established their westward supply lines, constructing a 37-mile (60-km) road from Wewak and creating a series of coastal defences to protect their water transportation efforts. Such supplies as there were had nonetheless to be manhandled forward by almost 7,000 troops. As of a time early in June, the 18^th Army had only half the ammunition Japanese logistics manuals specified as required for a major battle, and its men were receiving a mere half of their normal rations.

As early as a time late in May, Lieutenant General Walter Krueger, commander of the US 6^th Army, ordered reinforcements into the area in response to a range of intelligence about the Japanese build-up. Late in June, the Allies began moving Major General Leonard F. Wing’s US 43^rd Division from New Zealand, and Brigadier General Julian W. Cunningham’s 112^th Cavalry and the 124^th Infantry (the latter of Major General John C. Persons’s 31^st Division) arrived from eastern New Guinea. Throughout the period late in May, Japanese troops closed on the US outposts along the Dandriwad river, and after a series of clashes, early in June Adachi’s troops had forced the US forces to withdraw from the Yakamul area and rejoin the main body along the Driniumor river. By a time late in June, Allied forces in the area had been built up to corps strength, and Major General Charles P. Hall, commander of the XI Corps, had established his headquarters at Aitape. At about this time, a covering force centred on the 112^th Cavalry was sent despatched about 20 miles (32 km) to the east to guard Aitape’s eastern flank on the line of Driniumor river.

Despite these preparations, the Allied intelligence picture was both confused and contradictory. In the period immediately preceding the Japanese attack, early in July Allied patrols were unable to locate the Japanese troop concentrations. Both Hall and Brigadier General Charles A. Willoughby, MacArthur’s chief of intelligence, believed during June that the Japanese were incapable of conducting an attack. While they had access to decoded Japanese radio messages, which indicated that such an attack was imminent, both men regarded this as unlikely given that other Japanese messages also revealed the 18^th Army's desperate logistical situation. Additional intelligence, including information gained by fighting patrols, led to US troops being placed on alert in expectation of a Japanese assault on several occasions late in June and early in July. No attack eventuated, however, as the Japanese had been forced to delay the operation. When the attack began, therefore, the US forces were taken by surprise.

On the night of 10/11 July, about 10,000 Japanese attacked en masse across the Driniumor river. In support of this effort, the Japanese had moved several 70- and 75-mm (2.76- and 2.95-in) pieces of artillery forward through the jungle. The Japanese attack plan had envisaged commitment of the 78^th Regiment in the south, 80^th Regiment in the centre and 237^th Regiment in the north for simultaneous attacks in line abreast on a front between Paup and Afua. Following a five-minute artillery bombardment, the attack began at 22.55 on 10 July. The initial attack was poorly co-ordinated, being hampered by the nature of the terrain, which resulted in the 78^th Regiment launching its assault 20 minutes before the 80^th Regiment, which was followed by the main elements of the 237^th Regiment at about 02.00 on 11 July. By 03.00, the Japanese assault had petered out, having gained about 1,315 yards (1200 m). A secondary attack began at about 05.00 when follow-on elements of the 237^th Regiment, together with supporting medical, staff and artillery personnel, crossed the Driniumor river. This secondary movement continued until around dawn when elements of the two main Japanese assault units, the 78^th Regiment and 80^th Regiment, began the process of reconstitution on an area of high ground about 800 yards (730 m) to the north-west of the US line. The 237^th Regiment, whose commander, Colonel Nara, had become separated from his headquarters, took longer to reorganise.

Companies E and G of the 128^th Infantry bore the brunt of the combined efforts of the three assaulting Japanese regiments on 10/11 July. The US companies were supported by organic heavy machine gun and mortar fire, as well as indirect fire of the 120^th and 129^th Field Artillery Battalions, positioned in support along the eastern bank of the Driniumor river. The weight of this massed firepower inflicted heavy casualties on the Japanese attackers and destroyed large amounts of their equipment including machine guns and indirect-fire support weapons. Company G was largely able to hold its positions, anchored on the right by a supporting battalion of the 127^th Infantry, but Company E’s line in the centre collapsed under the pressure. At least 30 of the company’s men were killed or wounded, although some of the survivors, along with supporting detachments from Company H, were able to withdraw to Company F’s lines on the US left flank near the coast; others remained isolated behind the Japanese line for at least three days.

Despite their heavy casualties from machine guns and artillery, the Japanese troops in the initial assault drove forward and forced a major breach in the US line. In response, US forces began to pull back to delaying positions throughout 11/12 July in an effort to prevent further Japanese advances. During the initial assault, as noted above, the Japanese had succeeded in pushing through the centre of the US line, forcing a withdrawal of about 3 miles (4.8 km) to the west into an area around Koronal Creek and the X-ray river. The Japanese were unable to take full advantage of their initial success, however, as a result of their supply and communications problems. In response, US commanders ordered a counterattack, and throughout 13 and 14 July the US forces worked to restore their line, closing a gap that had developed between the northern and southern forces. In support of the US counterattack, the 120^th, 129^th and 149^th Field Artillery Battalions of 105-mm (4.13-in) howitzers were committed: the 129^th and 149^th Field Artillery Battalions were allocated to the northern force, while the 120^th Field Artillery Battalion supported the southern force. The fire of the 105-imm (4.13-in) howitzers was supplemented across the front by the that of the 181^st Artillery Battalion’s 155-mm (6.1-in) guns. After a fighting withdrawal through the jungle that night, the US defenders managed to regroup where possible and by 13 July were counterattacking to try to seal the breach. In support of this, two battalions of the 124^th Infantry were committed, disrupting the 237^th Regiment's investment of the Paup villages, and resulting in further clashes around Tiver.

The Japanese prepared a renewed attack by elements of the 20^th Division to the north-west of Afua from 15/16 July. This effort resulted in heavy clashes with elements of the 112^th Cavalry and 127^th Infantry on the Allies' southern flank, but failed to secure the village for the Japanese, who captured it at least twice before being compelled to withdraw.

After the first effort against Afua, the remainder of July saw heavy fighting in the area to the west of the river as platoons and troops, companies and squadrons, and battalions clashed in the jungle along the coast and around the Torricelli mountains. As the fighting devolved into hand-to-hand combat in the jungle, heavy pressure was maintained on some pockets of US troops still clinging to their positions on the river as they became enveloped by Japanese troops determined to destroy them out and secure the Afua area. By 22 July, the Japanese had captured Afua, but on the following day reinforcements from the 127^th Infantry started to relieve the isolated cavalrymen. In response, Adachi decided to launch another effort around Afua, committing his reserve, the 66^th Regiment, and the bulk of the 41^st Division to an all-out attack alongside the 20^th Division. Beginning on 29 July, and lasting several days, the attack took some ground but resulted in heavy Japanese casualties.

Meanwhile, US forces began preparing to launch a counter-offensive between 29 and 31 July. This effort was intended to outflank the Japanese forces attacking around Afua with elements of the 124^th Infantry and 169^th Infantry pushing to the east of the Driniumor river from the north of the Allied line near the coast, advancing to Niumen Creek before turning to the south and then to the west to envelop the Japanese forces attacking the US southern flank. By the beginning of August, the Japanese drive against Afua had petered out and the Japanese were eventually pushed back to the east over the Driniumor river. The right-flank Japanese forces near the coast then switched to the defensive, offering strong resistance before attempting to resupply and reorganise around Yakamul and Maljip. By 4 August, Adachi had ordered a complete withdrawal toward Wewak, although fighting lasted until about 10 August as US troops continued to clash with the Japanese rearguard. At around this time, elements of the 43^rd Division, comprising the 103^rd, 169^th and 172^nd Infantry, started to relieve the 127^th and 128^th Infantry. From 16 August the fresh regiments took up the pursuit of the withdrawing Japanese. During this time, US troops patrolled toward Marubian, Charov and Jalup, but were unable to re-establish contact until reaching strong positions on the Dandriwad river. At this point, Krueger called a halt to the advance and the battle was officially declared at an end on 25 August.

Air support during the fighting was provided to the Allied ground troops by Bristol Beaufighter twin-engined heavy fighters and Bristol Beaufort twin-engined bombers of the Royal Australian Air Force’s No. 71 Wing and US aircraft of the 110^th Reconnaissance Squadron operating from Tadji and Saidor. Naval gunfire support was provided by Task Force 74, comprising the Australian heavy cruisers Australia and Shropshire, the Australian destroyers Arunta and Warramunga and the US destroyers Ammen and Bache. Allied PT-boats and destroyers also interdicted Japanese barge convoys carrying supplies between Aitape and Wewak, and fired on troop concentrations along coastal avenues of advance. As a result of the density of the terrain and its vegetation, most supplies were airdropped to the US ground forces rather than being carried forward overland.

In total, the US forces had suffered almost 3,000 casualties including 440 men killed, 2,550 wounded and 10 missing, while the Japanese lost between 8,000 and 10,000 men including both battle and non-battle casualties, the latter as a result of starvation and disease. Of the US units involved in the battle, the 112^th Cavalry, 124^th Infantry and 169^th Infantry took the heaviest casualties. The four-week 'Battle of the Driniumor River' was one of the costliest of the campaigns in Papua and New Guinea, second only to the bloody head-on Allied assaults of the Japanese strongholds at Gona, Buna and Sanananda between November 1942 and January 1943.

In the aftermath of the 'Battle of the Driniumor River', the US forces focused their efforts largely on the defence of their base and airfield area around Aitape, undertaking only limited patrolling round the perimeter. Meanwhile, Adachi reorganised his forces and shifted his headquarters to Wewak. The 51^st Division also established itself there, while the 20^th Division reoriented its elements between But, Dagua and Maprik, and the 41^st Division moved to the area of the Anumb river and Balif. Having suffered heavy casualties and still receiving little in the way of supplies, the Japanese were now compelled to embark still further on subsistence operations. They subsequently clashed with Australian forces during the Aitape-Wewak campaign from a time late in 1944 after the Australians had arrived in the area to relieve US troops being transferred to the Philippine islands group.