Battle of Sio

Battle of Sio
Part of World War II, Pacific War
Australian troops of B Company, 30th Infantry Battalion crossing a shallow creek between Weber Point and Malalamai. They have been advancing from Roinji over muddy swamps, Kunai grass and innumerable rivers to link up with American troops at Yagomai.
Date 5 December 1943 – 1 March 1944 Location Sio, Papua New Guinea 05°57′18″S 147°22′12″E / 5.95500°S 147.37000°E / -5.95500; 147.37000 (Sio) Result Allied victory
Belligerents
Australia Papua  United States	Japan
Commanders and leaders
Douglas MacArthur Leslie Morshead Frank Berryman	Hatazō Adachi
Strength
~15,000[1]	~8,000[2]
Casualties and losses
83 killed 186 wounded[3]	1,421 killed 2,198 found dead 76 captured[3]

The Battle of Sio, fought between December 1943 and March 1944, was the break-out and pursuit phase of General Douglas MacArthur's Huon Peninsula campaign, part of the New Guinea campaign of World War II.

After the defeat of the Japanese in the Battle of Sattelberg, Australian Army forces broke through the Japanese positions around Finschhafen. Constant pressure from United States Navy PT boats, Australian land forces and Allied aircraft brought the Japanese logistical system to the brink of collapse, resulting in disease, malnutrition, and privation for the Japanese soldiers. Meanwhile, the Allied supply system grappled with the problems of terrain and climate, particularly inclement weather and rough monsoonal seas that hampered and occasionally prevented delivery of supplies by sea.

Australian and Papuan troops advanced along the coast of the Huon Peninsula, using infantry, tanks, and air strikes against the Japanese positions, which were generally sited at creek crossings in the jungle. The advancing infantry kept strictly within range of the supporting artillery, which was liberally employed in the early stages of the operation. Using tactics that exploited the firepower of Australian artillery and armour, the Australian and Papuan troops inflicted heavy and disproportionate casualties on the Japanese as they advanced, ultimately linking up with the American forces at Saidor. Hundreds of Japanese soldiers were killed; thousands more died from disease, malnutrition, exhaustion and suicide. The Allies failed to seize the opportunity to completely destroy the Japanese forces.

During the advance, Australian troops captured Japanese cryptographic materials. This had an important effect on the subsequent course of the war against Japan in the South West Pacific, as it permitted codebreakers in Australia and the United States to read Japanese Army messages on a much greater scale than previously. This major intelligence windfall led to MacArthur accelerating the South West Pacific's timetable by over three months, starting with the Admiralty Islands campaign in February followed by the landings at Hollandia and Aitape in April far behind enemy lines.