The Japanese had occupied northwestern Borneo since early in the Pacific War, seizing the vital oilfields that by 1943 were fueling their war machine. Allied efforts to reclaim the territory had been limited to aerial bombing while ground forces focused on the Philippines. But with Allied operations to capture Labuan and Brunei Bay scheduled for mid-June 1945, someone needed eyes on the ground. The Australian Services Reconnaissance Department launched Operation Semut with men who already understood Borneo. Major Tom Harrisson, a maverick anthropologist who had lived among the island's indigenous peoples, led Semut 1 into the Kelabit Highlands near Bario. Major G. S. Carter, a New Zealander serving in the Australian Army, commanded Semut 2. Captain W. L. P. Sochon took Semut 3 to the Upper Rajang. All three leaders possessed the language skills and cultural knowledge that no amount of combat training could substitute.

The operation's genius lay in its reliance on the Bornean peoples themselves. The Kelabit, Kayan, and Iban communities did not simply cooperate with the Australians; they became the operation's backbone. After Harrisson's team parachuted into the Kelabit Highlands, local laborers built a small airstrip at Bario, allowing resupply and further insertions. The Kelabit people helped transfer the Semut 2 team to the Baram Valley, where they established a base at Long Akah. Sochon's Semut 3 team traveled to Belaga at Upper Rajang with the full support of local Kayan and Iban communities. The relationship was genuine and reciprocal. These were not puppet forces manipulated by distant strategists. The indigenous communities had endured Japanese occupation and chose to fight alongside the Australians because liberation served their own interests as much as any Allied war aim.

The intelligence haul was extraordinary. All information gathered by the Semut teams was relayed to General Thomas Blamey's Advanced Land Headquarters at Morotai in the Halmahera. Operatives mapped Japanese troop dispositions, identified transportation routes and staging points, located airfields, ammunition dumps, and food supplies, and gathered information about Allied prisoners of war in the region. Days before the Allied landings around Labuan and Brunei Bay, Semut 2 captured a Japanese communications station at Long Lama. On 9 June 1945, the eve of the Australian landings at Labuan island, Semut 1 attacked a Japanese garrison at Brunei Bay. The operation even produced its own subchapter of dark comedy: Semut 4B, operating out of Mukah in August, saw Lieutenants Rowan Waddy and Ron Hoey paddling collapsible folboat canoes up the Mukah River, hunting for remaining Japanese holdouts, only to find themselves menaced by a crocodile roughly the length of their boat.

Japan's mid-August surrender did not end the work. Under Harrisson's command, Semut operatives continued operations around Sapong until late October 1945, working to secure the capitulation of Japanese troops still engaged in fighting with local Bawang guerrillas. Some Japanese soldiers, cut off from communication and unwilling to believe the war was over, had to be convinced individually. Assisted by the Semut and companion Operation Agas, the Australian 9th Division secured north Borneo, with major combat ending largely by July. As regular forces held the coast, Japanese units retreated inland, where Semut's irregular forces called in airstrikes on the withdrawing columns and helped restore civil administration. Extensive civic reconstruction began even before the formal end of hostilities: rebuilding oil facilities, establishing schools, providing medical care to local civilians, and restoring water supplies.

Historian Ooi Keat Gin, writing in the Australian War Memorial Journal, concluded that Operations Agas and Semut achieved considerable results within a short period, greatly assisting the wider Allied effort to secure north Borneo. The assessment was not entirely uncritical. Intelligence relayed by the local population was sometimes inaccurate, and Allied interrogators failed to assess the reliability of information before passing it to headquarters. Rumors circulated as freely as facts, and Japanese counter-intelligence efforts prevented the insertion of European officials to supervise the network. But these were the imperfections of an operation that succeeded on its core terms. The 1969 novel Farewell to the King by Pierre Schoendoerffer drew on Harrisson's exploits, and John Milius later adapted it into a film. The real legacy, though, lives quieter. In the longhouses of the Kelabit Highlands, the memory of the Australian commandos who dropped from the sky and trusted the people of the forest remains part of the oral history, a rare wartime alliance built on mutual respect rather than compulsion.