The cost of building a major naval base at Milne Bay was measured partly in sweat and partly in malaria. The shore of the bay, where it was not mangrove, was wet tropical jungle that had to be cut, drained, and graded before anything could be constructed on it. The Seabees of the 55th Battalion landed first, starting work on 23 May 1943 at Kana Kopa on the southern shore. Within weeks they had built a PT boat advance base with an engine-overhaul shop, a camp for 800 troops, a supply depot, water tanks, a wharf, and pontoon drydocks. The 84th Battalion followed. Then the 91st. Then the 105th and Amphibious Construction Battalion 2. By the winter of 1943-44, Seabees had turned the bay into a working naval installation the length of a medium-sized American city - and had done it while the tropical climate did its patient best to reclaim every inch.

The PT boats were what Milne Bay did first. US Navy PT Boat Advance Base Six opened at Kana Kopa on 11 December 1942, on a site that Australian naval forces had already used as an anchorage during the August battle. Through 1943 and into 1944, the base supported raids on Japanese-held positions at Salamaua and Nassau Bay, with USS Oyster Bay, USS Hilo, and USS Wachapreague serving as PT boat tenders. A second PT base operated on Fergusson Island to the north. The crews lived in mosquito netting and rotating fevers, and the boats went out at night, across waters where the charts were often wrong and the enemy often invisible. In June 1944, as the war moved forward, the PT boat operation was relocated to Dreger Harbour near Finschafen. The base at Kana Kopa kept running, but its center of gravity had already shifted north.

At Swinger Bay, inside what is now Alotau, Amphibious Construction Battalion 2 and the 105th Seabees built something less remembered but arguably more consequential: a US amphibious training center. Seabees graded a massive site, put up Quonset huts for 1,500 troops, laid out classrooms, built a 2,000-foot waterfront, and opened the center in January 1944. It trained American soldiers and Marines for the beach landings that would define the rest of the Pacific War. The US Army's 32nd Infantry Division trained here. The 41st Infantry Division trained here. The 1st Marine Division - the Guadalcanal veterans - also passed through, rehearsing the amphibious assaults they would carry out at Peleliu and Okinawa. The Seventh Amphibious Force itself moved from Toorbul, Queensland, to the Alotau base in 1944, using it as an operational headquarters until March 1945, when it shifted forward to Subic Bay in the Philippines.

Human cost demanded its own infrastructure. At Hilimoi Bay, five miles east of the main depot at Gamadodo, Amphibious Construction Battalion 2 built a 500-bed naval hospital and a 3,000-bed rest and recovery center. At Gili Gili Dock, a jetty and pontoon wharf handled loading and unloading. At Gohora Bay near Ladava, Seabees built a destroyer repair base. An auxiliary floating drydock - built by the Pacific Bridge Company - served Milne Bay as USS Fulton serviced its submarines. The scale adds up quickly. By mid-1944, Milne Bay was a small maritime city, with three airstrips (Gurney, Turnbull, and the abandoned Waigani), a submarine repair unit, an ammunition depot, supply depots at Gamadodo and Gili Gili, and the network of PT bases, seaplane bases, and training camps strung around the bay. It was also, for the wounded Marine or Army soldier arriving by hospital ship from Guadalcanal or Cape Gloucester, the first place that felt like rear area.

By July 1945, the war had moved too far north. Most of Naval Base Milne Bay was packed up and shipped to Subic Bay and Okinawa, and what the Seabees left behind was gradually absorbed by the landscape and the towns. Turnbull Field, No. 3 Strip, became a memorial park. Gurney Airport is still in service. Waigani Airfield, abandoned during the war because it never drained properly, has been reclaimed entirely by swamp and forest. In Alotau today, the Turnbull War Memorial Park holds three monuments to those killed in the August 1942 battle - improved and rededicated by Australia in 2002, with a battlefield relief map showing where the fighting happened. The Milne Bay War Memorial stands at the Masurina Lodge. Further south, in Queensland, the Milne Bay Memorial Library and Research Centre continues to collect the records of what the Seabees and the Australians and the US Marines did here. The harbor that they all needed is still twenty miles long. It is quieter now. The freighters that tie up at Alotau are mostly carrying copra and oil palm.