The Battle of Slater's Knoll (28 March – 6 April 1945) was fought between Australian and Japanese forces on Bougainville Island during the Second World War. Part of the Bougainville campaign, the battle occurred as a force of about 3,300 Japanese from the Japanese 6th Division, including artillery and other supporting elements, launched a counterattack against the main Australian offensive which had been pushing south towards Buin, concentrating their attacks on Slater's Knoll near the Puriata River. The Australian troops belonged to the 7th Brigade, with the 25th Infantry Battalion being the most heavily engaged, although the 9th Infantry Battalion and the 61st Infantry Battalion also took part in the fighting.
Against Japanese tactics that included massed attacks, the Australians utilised armour and artillery, and in the end these proved decisive. Commencing in late March, after the Australian advance had been halted by wet weather, over the course of several days the Japanese launched several probing raids followed by heavy attacks against the Australians. The final assault on the knoll came on the night of 4/5 April when 129 men from 'B' Company, 25th Infantry Battalion repulsed an attack by a force of about 1,100 Japanese, killing 292. This proved to be the 7th Brigade's final involvement in the campaign as they were relieved by the 15th Brigade shortly afterwards. Overall, 620 Japanese were killed in the battle with another 1,000 estimated to have been wounded, while the Australians suffered 189 casualties.
At the beginning of 1945, the south-eastern part of Bougainville was occupied by the Japanese 6th Division, under the command of Lieutenant General Tsutomu Akinaga. Made up of the 13th and 23rd Infantry Regiments—600 and 700-strong respectively—as well as the 6th Field Artillery Regiment, the 6th Engineer Regiment, the 6th Transport Regiment and the 4th Field Heavy Artillery Regiment, Akinaga's division consisted of 3,300 men.
The previous November, the Australian II Corps under Lieutenant General Stanley Savige had taken over responsibility of the island from the US XIV Corps, who had been transferred to fight in the Philippines. Although incorrect, at the time the Allies believed that the Japanese forces on the island numbered around 17,500 men and while these forces were understrength, they were still considered to be capable of carrying out effective combat operations. In order to counter this, it was decided that the Australian II Corps—consisting of Militia and Australian Imperial Force troops from the 3rd Division and the 11th and 23rd Brigades—would go on the offensive and a three-pronged campaign was planned in the northern, central and southern sectors of the island.
As a result, the Australian campaign on the island developed into 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 thoroughfares and protection against further counterattacks, while also opening the way for a drive to the east coast; and the main campaign would take place in the south, where the bulk of the Japanese forces were concentrated. Consequently, on 21 January, Savige directed the 3rd Division under Major General William Bridgeford to take "swift and vigorous action" to destroy Japanese forces in southern Bougainville. The division's immediate task was to advance south to the Puriata, and send patrols across it. Savige considered that the Japanese division was "weak and off balance" due to casualties and disease as well as a shortage of supplies. Brigadier Raymond Monaghan's 29th Brigade had opened the campaign in the south in mid-December, crossing the Jaba River, before conducting a series of landings by barge along the coast of Empress Augusta Bay, outflanking the Tuju, Tavera, Adele and Hupai Rivers, in what became dubbed the "Battle of the Swamps" by the Australians. The fighting during this time resulted in 240 Japanese killed, and 148 Australian casualties. Monaghan was also relieved of his command.
In late January, the 7th Brigade, under Brigadier John Field, which had been patrolling the Jaba River to protect the 29th Brigade's rear while they pushed south, was subsequently tasked to "take Mosigetta, clear the enemy from the Kupon–Nigitan–Sisiruai area, and patrol along the Puriata". Each of the brigade's three infantry battalions was marginally below full strength at the time, with frontages of between 600 and 700 personnel around the start of the battle.
In late January, a company from the Australian 25th Infantry Battalion—under Lieutenant Colonel John McKinna—subsequently carried out an amphibious landing south of Motupena Point, on the Solomon Sea coast and had cleared towards Matsunkei. On 2 February, the battalion carried out another landing at Toko, and advanced northeast from there towards Batara, and then along the west bank of the Puriata, towards the Buin Road. Meanwhile, further to the east, the 9th Infantry Battalion was moved by truck from Torokina to the Jaba River and then moved by barge along the Empress Augusta Bay coast, landing at Mawaraka from where they advanced towards Mosigetta, and the 61st Infantry Battalion also proceeded towards there, setting off from the junction of the Jaba and Pagana Rivers while the 2/8th Commando Squadron provided flank security further to the east. Elements of the 1st New Guinea Infantry Battalion also conducted a landing on the coast, on the 25th Battalion's left flank, to the north of Toko, and advanced to Makaku and then towards Mosigetta.
On 4 March, a company from the 25th Infantry Battalion crossed the Puriata River southward—at the point where the Buin Road crossed—on a north-facing bend, later known as Galvin's Crossing. The following day the Japanese were forced off a small knoll close to the river and the road, about 1 km (0.62 mi) south of the crossing. On 6 March, Japanese shelling of the knoll wounded an Australian, Private Carl Slater, who held his post until relieved. He was the only casualty during this period, and the knoll was named after him. Throughout March, the 25th Infantry Battalion continued to expand its perimeter, to the north and south, along the Buin Road, while the 9th moved west from Mosigetta to link up with them and the 61st skirted east around Makapeka. By late March, though, heavy rain temporarily stopped the Australian advance, as the main road became impassable due to thick mud.
Although the Japanese sporadically shelled the knoll during March, there were no further Australian casualties. The 25th Infantry Battalion, which had established its headquarters and main defensive position around the knoll, in front of the Puriata River, sent out patrols every day, and on 9 March, one of these reported having killed 10 Japanese soldiers without loss to itself. The Japanese also began making dawn attacks, in which light machine gunners would approach by stealth, set up and fire on the Australian positions, then retire to their own lines, 250 yd (230 m) away. As the advance was halted, the Australians began to expect a major attack in the area of the knoll.
The Japanese launched a number of attacks on the Australians between 15 and 17 March. Under fire from three sides and in danger of being enveloped, on 19 March the Australians launched an attack of their own along the Buin Road, and although they had some success in clearing the forward Japanese positions, they then encountered a system of pillboxes and trenches around the Hatai Road junction. The 25th Infantry Battalion's commander, McKinna, ordered the two platoons led by Lieutenant Dick Jefferies to attack the pillboxes, supported by a section of machine guns, and several PIAT anti-tank weapons. A two-hour fire-fight and a bayonet charge followed. This attack was ultimately beaten back by the defenders with the loss of eight Australians killed and 14 wounded. A second attack was launched on 22 March. Supported by heavy artillery, mortars and machine guns, it proved successful, largely after the individual efforts of Corporal Reg Rattey, who single-handedly silenced several bunkers.