By 1945, the Bougainville campaign was what military historians sometimes call an unnecessary war - the main Allied thrust had moved far to the north, toward the Philippines and Okinawa, and the Japanese garrison on Bougainville could have been left to wither. But in November 1944 the Australian II Corps under Lieutenant General Stanley Savige took over the island from the American XIV Corps, and Australia was not going to sit still. Savige planned three drives: contain Japanese forces in the north on the Bonis Peninsula, seize Pearl Ridge in the center to control the east-west routes, and press south toward Buin where the bulk of the Japanese forces sat. In southern Bougainville, the Japanese 6th Division under Lieutenant General Tsutomu Akinaga held the line - 3,300 men on paper, including the 13th and 23rd Infantry Regiments, the 6th Field Artillery Regiment, and other supporting units. Disease, supply shortages, and isolation had worn them down, but they were still capable of attack. The Australians underestimated their numbers. The Japanese underestimated Australian strength. Both sides were about to learn.

The Australian 7th Brigade, under Brigadier John Field, pushed south with three battalions - the 25th under Lieutenant Colonel John McKinna, plus the 9th and the 61st. Through February and early March, they conducted landings, marched through mud, and fought small actions along the Puriata River. On 4 March, a company from the 25th crossed the Puriata at what became known as Galvin's Crossing, and the next day forced the Japanese off a low rise about a kilometer south. That hill - later Slater's Knoll - became the battalion's forward base. McKinna established his headquarters and main defensive position around it. Patrols went out daily. Japanese gunners occasionally shelled the position. And on 6 March, Private Slater held through the shelling and gave the knoll its name. Then in mid-March came Corporal Reg Rattey, who during an attack on Japanese pillboxes on 22 March single-handedly silenced several bunkers - an action that would later earn him the Victoria Cross, awarded for conduct that even by the standards of infantry combat was extraordinary. Heavy rain halted the advance in late March. The Japanese, believing the Australian force around the knoll was only 400 men strong, decided to counterattack.

The attack began in pieces. On the night of 27/28 March, a hundred Japanese soldiers fixed bayonets and assaulted the gap between the 25th Battalion's main position and its rear echelon - but the Australians had been alerted when the communication line was cut, and the weapons pits were already manned. The attack failed. Nineteen dead Japanese were counted in the morning. Three Australians were killed, seven wounded. The next night brought an attack on the 9th Battalion's rear echelon at Barara, where four Bren light machine guns held off a hundred attackers until dawn. On 30 March, a Japanese force hit one of the 25th's forward companies four times, finally forcing an Australian withdrawal that left three mortars and ammunition behind - weapons the Japanese promptly used against their former owners. Communications broke down. Water ran out. A well was dug inside the perimeter. Major Kenneth Arnott's squadron of Matilda tanks from the 2/4th Armoured Regiment was pulled up from Toko, with engineers bridging creeks and bulldozers hauling tanks through mud. By 19:00 on 30 March, the tanks reached the rear echelon. By 31 March they were on the knoll itself. It was the first time armor had been used in the Bougainville campaign, and it would make the difference.

The Japanese divisional commander pulled his regiments back across the Puriata on 2 April and scheduled a fresh attack for 5 April. Three days of quiet let the Australians fortify. They laid barbed wire, cleared fields of fire, set booby traps, registered their Vickers medium machine guns and Bren light machine guns on carefully surveyed engagement areas to the north, northwest, and south. On the night of 4/5 April, the Japanese cut the signal lines and started shelling. B Company of the 25th Infantry Battalion - 129 men - stood to on Slater's Knoll. At 05:00 on 5 April, 900 to 1,100 Japanese soldiers from the 13th and 23rd Infantry Regiments attacked in waves, first from the north and almost immediately from the southwest. The assault overwhelmed the Australian forward positions, then ran into the wire. The attackers had brought no equipment to deal with wire obstacles. They piled up against it. Australian artillery fire and machine gun fire cut them down in swaths. Another attack went in along the Buin Road against other forward companies. It too was broken. In eighty minutes of combat, the Australian defense held - and when the charge failed, some Japanese officers prepared for one last suicide attack. Colonel Toyoji Muta of the 13th Infantry Regiment wrapped his unit's banner around his waist and armed himself with a grenade to blow himself up in the final rush. The officers of the 23rd Infantry made a similar pact. But Akinaga, realizing further assault was futile, canceled the attack. The charge never came. By 08:30 the Japanese had begun to withdraw.

When the sun rose, Australian soldiers counted 292 Japanese dead in the wire around B Company's perimeter. Over the whole course of the battle, the Australians counted 620 Japanese killed and estimated another 1,000 wounded. The commander of the 23rd Infantry Regiment, Lieutenant Colonel Kawano Koji, was among the dead. So was Lieutenant Colonel Honda Matsuo, a senior divisional staff officer. Four wounded Japanese soldiers were taken prisoner. Australian casualties came to 189 - 10 officers and 179 other ranks killed or wounded. Japanese soldiers who survived wrote later of the effect on morale. They were lacking food, suffering from disease, isolated from home. Many realized the war was lost. Desertions increased. Most kept fighting only because they were ordered to. The battle was the 7th Brigade's last major action in the campaign - they were relieved by the 15th Brigade shortly afterward. After the war, the battle honor Slater's Knoll was awarded to the 25th Infantry Battalion and the 2/4th Armoured Regiment. Reg Rattey's Victoria Cross was announced in the London Gazette in July 1945.

Slater's Knoll today is a small rise in the forests of southern Bougainville, near where the Buin Road crossed the Puriata River. The Japanese troops who attacked it are buried in unmarked graves that the jungle has long since reclaimed. The Australian 25th Infantry Battalion has been disbanded for decades, but the battle honor still appears on unit histories. Private Carl Slater, whose name went onto the map by accident, held his post at the right moment and gave the place its identity. Lieutenant General Masatane Kanda, commanding the Japanese XVII Army, had been so confident of victory that he developed no plans for what came after. There was no after. The battle did not end the Bougainville campaign - that would drag on through July when heavy rain and flooding finally stopped the fighting - but it broke the back of Japanese resistance in the south, and it was a reminder of how much can turn on a single hill and a single company of infantry, when 129 men are ready for what is coming and 1,100 are not.