The campaign began because Lae had fallen faster than anyone expected. Major General George Vasey's 7th Division, dropping in at Nadzab and pushing down the valley, met Major General George Wootten's 9th Division coming from the coast. The pincer closed on 16 September 1943, but heavy rain and the shape of the ground let much of the Japanese garrison slip north and inland. Someone had to follow. The 9th Division turned east onto the Huon Peninsula. Vasey's 7th turned west, up the Markham, chasing a retreating enemy through country where the grass grew taller than a man and every patch of shade might hide a rifle. At Kaiapit, the 2/6th Commando Squadron caught a Japanese column and killed more than two hundred in a single engagement. After that, resistance thinned. By early October the Australians were in Dumpu.

The prize at the end of the valley was not a town. It was a line of airstrips. Control the Ramu Valley and you could build bases at Gusap from which Allied aircraft could reach the great Japanese anchorages at Wewak and beyond. The 7th Division spent much of the autumn building and guarding those strips while the 6th Machine Gun Battalion came up from Port Moresby to defend Gusap. But the Japanese held the high ground. On the northern wall of the valley rose the Finisterres, and within them, above the Ramu river, sat Kankiryo Saddle and the 1,500-metre spine the Australians had already named Shaggy Ridge. So long as the Japanese sat up there, they could see everything. They were also building a road south from Madang through Bogadjim, aiming to punch back into the valley and retake what they had lost. For Vasey, that decided it. The mountains had to be taken.

The fighting in October around Palliser's Hill and John's Knoll was only the beginning. When Brigadier Ivan Dougherty's 21st Brigade pushed onto John's Knoll, a single understrength platoon held off three companies of Japanese infantry supported by artillery and heavy machine guns. Casualties were seven Australians killed and twenty-eight wounded against two hundred Japanese dead. In November the 25th Brigade relieved the 21st. In December the offensive opened in earnest against Shaggy Ridge itself, a 6.5-kilometre razorback scattered with rocky outcrops the Japanese had fortified into strongpoints. The only way forward was single file along the spine. On 27 December 1943, troops of the 2/16th Infantry Battalion watched aircraft bomb a feature called The Pimple and then climbed up to take it themselves. The battle ground on into January. When Shaggy Ridge finally fell, the Japanese began to withdraw toward the coast.

While Vasey's men fought in the mountains, an American force had been grinding along the coast. On 2 January 1944, the US 32nd Infantry Division had landed at Saidor, cutting the coastal track behind the retreating Japanese. What followed was a double squeeze: Australians pressing north through the ranges, Americans pressing west along the shore. The Japanese 18th Army under Lieutenant General Hatazo Adachi made the only choice it had left. It abandoned the coast and marched overland, across flooded rivers and trackless ridges, trying to reach Wewak. Many men did not arrive. Starvation, disease, and the Finisterres themselves did what bullets could not. Madang fell on 24 April 1944 to the 8th and 15th Brigades. The 30th Infantry Battalion took Alexishafen the next day. A follow-up landing on Karkar Island closed the map.

The Australian Army later awarded battle honours for the campaign: Ramu Valley, Shaggy Ridge, Finisterres, Barum, Bogadjim, and Madang. Six names for seven months of mountain climbing under fire. Flying over the area now, the corridor is still obvious from altitude, a long green trough walled by cloud-catching peaks. The airstrips at Gusap and Nadzab, built and defended at such cost, still show in the ground. Shaggy Ridge is a harder thing to pick out from the air, just another dark spine among many, which may be the most honest memorial. The men who took it would have been the first to say the mountain was more formidable than anything they built on it.