Part of the Bougainville Campaign of the Pacific Theater (World War II)
The Battle of the Hongorai River took place during the Second World War and involved Australian, New Zealand and Japanese forces. Part of the wider Bougainville Campaign of the Pacific theatre, the battle was fought in the southern sector of Bougainville Island. Coming after the Battle of Slater's Knoll in which a strong Japanese counterattack was defeated, the battle occurred in two distinct periods between 17 April and 22 May 1945, as elements of the Australian 15th Brigade advanced south along the Buin Road.
The initial phase saw the Australians advance towards the Hongorai River. Following the end of the early fighting, the Australian advance towards the main Japanese concentration at Buin continued as they struck out towards the Hari and Mivo Rivers. This continued until torrential rain and flooding brought the advance to a halt short of the objective, washing away many bridges and roads upon which the Australians relied for supplies. As the Australian advance stalled, the Japanese began harassing the Australian line of communications, and as the rain stopped and the flooding subsided in late-July and into August, the Australians began making preparations to resume the advance towards Buin again. Ultimately, though, the war came to an end before the final Australian advance began, bringing the campaign to an end.
 ;templatestyles src="Template:TOC limit/styles.css" / ;
Background.
Strategic situation.
Japanese forces had landed on Bougainville in early 1942, capturing it from the small force of Australians garrisoning the island. They had subsequently developed several airbases on the island, using it to conduct operations in the northern Solomon Islands and to attack the Allied lines of communication between the United States, Australia and the Southwest Pacific Area. These bases also helped protect Rabaul, the major Japanese garrison and naval base in Papua New Guinea, and throughout 1943, Allied planners determined that Bougainville was vital for neutralising the Japanese base around Rabaul. 
US Marines conducted an amphibious landing at Cape Torokina, on the western coast of the island, north of Empress Augusta Bay, in November 1943. After an initial counter-attack, the US Marines had been replaced by a garrison of US Army troops who began consolidating their position around Torokina, establishing a strong perimeter. In March 1944, the Japanese launched a heavy counter-attack, which was turned back with heavy casualties. After this, the situation on Bougainville became largely static, as the Japanese focused primarily on subsistence, and the US forces chose to adopt a mainly defensive posture focused on maintaining the perimeter around Torokina.
In late 1944, as part of plans to free US troops up for the Philippines campaign, the Australian II Corps—consisting of mainly Militia troops under the command of Lieutenant General Stanley Savige—took over responsibility for Allied operations on Bougainville from the American XIV Corps. Australian forces began arriving on the island between November and December 1944, initially establishing themselves around the US base at Torokina. Due to inaccurate intelligence, Savige mistakenly believed that the Japanese forces on the island numbered just 17,500 men, and he consequently decided that the Australians would pursue an aggressive campaign to clear the Japanese from Bougainville in order to free their troops for subsequent operations elsewhere, rather than maintaining the defensive posture the US forces had adopted. However, Allied estimates of Japanese strength were later found to be grossly inaccurate and after the war it was found that the number of Japanese on the island at this time was closer to 40,000.
The campaign that Australian planners developed entailed three separate drives: in the north, it was planned that Japanese forces would be forced into the narrow Bonis Peninsula and contained; in the centre the seizure of Pearl Ridge would give the Australians control of the east–west avenues of approach, as well as affording them protection against further counter-attacks, while also opening the way for a drive to the east coast; and the main campaign in the south, where the bulk of the Japanese forces were concentrated around Buin.
Preliminary moves.
Following the capture of Pearl Ridge in December 1944, the Australian 7th Brigade had been moved south and allocated to the drive towards Buin. In late March and early April 1945, they had fought the Battle of Slater's Knoll after which a brief lull followed as the Australians paused to shorten their supply lines. Meanwhile, the survivors of the Japanese force, heavily demoralised by their defeat, withdrew towards the Hongorai River. The Australian 3rd Division was then ordered to resume its advance south, being tasked with capturing the Hari River, while the Hongorai was also included as an "intermediate objective". Japanese strength in the southern sector was estimated by the Australians at about 10,500 men, of which 2,300 were believed to be directly opposing the 3rd Division. The 15th Brigade was considered to be the most experienced of the Australian units on Bougainville at the time and was moved up to relieve the 7th Brigade, which was in need of rest.
Under the command of Brigadier Heathcote Hammer the 15th Brigade consisted of three infantry battalions as well as two troops of tanks from the 2/4th Armoured Regiment, engineers from the 15th Field Company, a battery of 155-mm guns from 'U' Heavy Battery, field artillery from the 2nd Field Regiment, and a number of smaller support units. The 58th/59th Infantry Battalion took over responsibility for Slater's Knoll replacing the 25th Infantry Battalion while the 24th Infantry Battalion took up a position across the Buin Road. The brigade's third battalion—the 57th/60th Infantry Battalion—did not join them until the beginning of May, and so the 7th Brigade's 9th Infantry Battalion continued patrolling operations north of the Huio River, in the Rumiki area, until the 57th/60th could dispatch elements to relieve them. Further inland, the 2/8th Commando Squadron advanced in a wide arc to the south-east, defending the brigade's left, or eastern, flank.
The Japanese forces opposing the Australians belonged to the 6th Division, under the command of Lieutenant General Tsutomu Akinaga. Akinaga had been ordered to delay the Australian advance between the Hongorai and the Hari for as long as possible, and with these orders in mind he had installed a number of strong points along the Australian's expected line of advance. The division's infantry had suffered heavily in the previous battle around Slater's Knoll and as a consequence, several units had to be reorganized or amalgamated. The front line positions were assigned to the 6th Field Artillery Regiment, while the 13th Infantry Regiment was to hold five strongpoints to their rear along and astride the Buin Road, designated 'A' through to 'E'. The 6th Field and 4th Field Heavy Artillery Regiments both held strongpoints further back, designated 'F' and 'G'. The 23rd Infantry Regiment was placed in the rear, where it was being reconstituted following losses suffered during the attack on Slater's Knoll.
Battle.
Advance to the Hongorai.
The 15th Brigade took over the forward positions from the 7th Brigade on 17 April. Initial dispositions had the 24th Infantry Battalion on the Buin Road around Kero Creek, with the 58th/59th around Barara, north-east of Slater's Knoll, and the 57th/60th, when it arrived to relieve the 9th, would be positioned further east astride a secondary, parallel track known to the Australians as the Commando Road. Two days later, Hammer received the order to commence the advance towards the Hongorai from Savige, who offered him the support of the 29th Brigade as a mobile reserve in case of sudden counter-attack. In a change to the tactics that the Australians had previously employed prior to the fighting around Slater's Knoll, from early May they advanced on a two-battalion front, instead of one. The 24th Infantry Battalion was in the van, moving along the Buin Road with the 58th/59th protecting its flank and rear; while further inland the 57th/60th Infantry Battalion, commencing on 3 May, advanced along the Commando Road from Rumiki, after taking over from the 9th Infantry Battalion.
Moving forward under a creeping barrage as they moved beyond Tokinotu, the 24th Infantry Battalion was the first to contact the Japanese, carrying out an attack against Japanese positions around Dawe's Creek on 17 April. Supported by a troop of Matilda tanks from the 2/4th Armoured Regiment, an artillery barrage which fired over 700 shells, two infantry companies—'C' and 'D'—from the 24th attacked the position while another—'A' Company—carried out a flanking manoeuvre to cut another track further north towards Kindara and Hatai. The left forward company—'D' Company—reached its objective without trouble; however, 'C' Company—on the right along with the troop of tanks—came up against stiff Japanese resistance and became bogged down. 'A' Company also became embroiled in heavy fighting along the Hatai track. In support of 'A' Company, Matildas came forward and raked the jungle, hacking through the undergrowth to reveal several Japanese pillboxes, which were destroyed by the Australian armour. As night fell, 'C' Company dug in before resuming the attack the next morning. Engineers were brought forward, as was a bulldozer, and the gap was bridged. Amidst heavy fighting, the Australians forced their way across the creek. By the time that the position had been taken in the afternoon and the infantry had advanced to the line of exploitation beyond the creek, 37 Japanese had been killed for the loss of seven Australians killed and 19 wounded. After this, the Australians continued their advance towards Sindou Creek, which was a further to the southeast. In response, the Japanese launched a number of determined counterattacks over the course of the following week, although these were turned back. During this time, the Australians sent a number of patrols out in front of their forward elements, one of which managed to slip through the Japanese defensive positions either side of the Buin Road and carried out a reconnaissance of the Hongorai River about south of the main crossing. Further patrols were carried out, as well as a number of ambushes, before the advance was resumed on 26 April.
Resuming their advance, the Australians were supported by three squadrons of Corsairs from the Royal New Zealand Air Force—Nos. 14, 22 and 26 Squadrons—which bombed and strafed the ground in front of the advancing infantry, as well as a creeping barrage of artillery and mortar fire. With such strong support the Japanese offered little resistance and over the course of two days the 24th Infantry Battalion covered almost a third of the distance to the Hongorai, for just one man wounded. Further progress was made over the next week, but on 4 May the advance was slowed when they encountered a roadblock defended by a field gun along with a machine gun, mines and other improvised explosive devices. After this, the 15th Brigade's engineer support were called upon to regularly carry out route clearance and proving operations as the Japanese became increasingly desperate to destroy the Australian armour, to the extent that they were prepared to sacrifice an artillery piece in order to lure the Australian tanks into a trap where they could be destroyed by mines. The Japanese began to adapt their tactics in other ways also. To negate the effectiveness of the Australian tanks, the Japanese began to position themselves off the roads, forcing the infantry to fight without their armoured support. Additionally, the Japanese began to concentrate their artillery with increasing accuracy upon the advancing infantry, which they kept under constant observation and fire.
Meanwhile, the previous day, 3 May, the 57th/60th Infantry Battalion had begun operating along the parallel Commando Road to the north. Here they had a number of encounters and suffered casualties due to their inexperience in patrolling, which resulted in them being ambushed. They also kept up a steady advance and eventually beat the main force in reaching, and crossing the river, arriving there on 6 May. On 5 May, along the Buin Road, the 24th Infantry Battalion had pressed forward again. Advancing with a tank troop in support, they came up against a concealed field gun defended by approximately 100 Japanese. After the lead Matilda's machine gun jammed, the field gun opened fire on it, damaging it and wounding its crew. Moving around the stricken tank, the second Matilda, armed with a howitzer, opened fire and destroyed the field gun before sweeping the Japanese defenders from the position. That night, the Japanese artillery opened up on the Australian position with a heavy barrage,