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.
Background
General Douglas MacArthur's Operation Cartwheel began with spectacular victories in the landing at Lae and the landing at Nadzab but then faltered in the face of inclement weather, unfavourable terrain, and—above all—tenacious and aggressive Japanese opposition on land and in the air.Coates, Bravery Above Blunder, p. 148 The initiative passed to Major General Hatazō Adachi's Eighteenth Army which launched a series of counter-attacks against Major General George Wootten's 9th Division in the Battle of Finschhafen.Coates, Bravery Above Blunder, pp. 149–177 At the Battle of Sattelberg, Wootten finally inflicted a crushing defeat on Adachi.Coates, Bravery Above Blunder, p. 227
Although beaten, the Japanese did not leave the area. Lieutenant General Shigeru Katagiri, the commander of the Japanese 20th Division, ordered the 80th Infantry Regiment to hold the Wareo area to protect the withdrawal of the 79th Infantry Regiment and other units. The 2nd Battalion, 238th Infantry Regiment was to act as a rearguard on the coast.Coates, Bravery Above Blunder, p. 236 Lieutenant General Frank Berryman, the commander of Australian II Corps, now urged Wootten to commence a coastal advance to cut the Japanese supply lines and force Adachi to retreat from the Huon Peninsula if he were not already doing so. Wootten took a more cautious approach.Coates, Bravery Above Blunder, pp. 228–229 The Battle of Wareo proved that the Japanese intended to defend the area. After a fierce fight, Wootten managed to drive the Japanese from the high ground around Sattelberg and Gusika. In early December, Adachi ordered all his troops to withdraw to Sio. Wareo was captured by the Australians on 8 December and the last Japanese rearguards left the area on 15 December.Dexter, The New Guinea Offensives, pp. 675–679 Meanwhile, Berryman's coastal advance had commenced on 5 December.
Prelude
Offensive against the Japanese supply system
Early in October 1943, a special staff was established at II Corps Headquarters to study the Japanese supply system. It did not include an officer with experience in maintaining a large force over a native carrier line, and therefore took time to realise that the Japanese force could not be maintained over an inland track, as was first assumed. Operations soon confirmed that the Japanese were dependent on a coastal supply line. During the Battle of Sattelberg, the Allies set out to cut this supply line. A three-pronged approach was taken: Collecting centres for native foodstuffs and tracks leading from the coast were bombed by the US Fifth Air Force. This reduced food stocks available to the Japanese, and also drove away the native carriers that the Japanese depended on to carry their supplies up from the coast. PT boats of Task Group 70.1 attempted to interdict barge traffic along the coast by night, while Fifth Air Force fighters conducted sweeps for barges by day. The land forces attempted to physically cut the Japanese supply lines. The 9th Division captured Pabu, thereby cutting the most convenient inland track, while the landing on Long Island occupied an important barge staging area.Dexter, The New Guinea Offensives, p. 731
By December, the pitiful condition of Japanese prisoners confirmed for the Australians that "the Japanese logistic system was in the final stages of breakdown". Between 9 and 13 December, PT boats sank 23 barges, most of them south of Sio. On 7 January, the PT boats also attacked a submarine. No less than twelve barges were destroyed on the night of 8 January, one of which was loaded with ammunition and another with around 70 troops. On 9 January, PT boats attacked a group of six barges, which attempted to fight back. One barge was seen to sink. Another patrol engaged eight barges and destroyed two. A third patrol found six barges on a beach and destroyed them. Then on 10 January, three PT boats sank three troop-carrying barges, taking one Japanese prisoner. The same night, two barges were also sunk north of Sio Island.Dexter, The New Guinea Offensives, p. 734 When General Berryman saw Vice Admiral Thomas C. Kinkaid, the commander of the Allied Naval Forces, on 14 December, he made a point of congratulating him on the work his PT boats were doing.Berryman diary, 14 December 1943, Australian War Memorial: AWM93 50/2/23/331
Lieutenant General Tsutomu Yoshihara, Chief of Staff of the Japanese Eighteenth Army recalled:
Tactics and logistics
The main Australian advance was by infantry-tank-engineer teams moving along the coastal tracks.Coates, Bravery Above Blunder, p. 243 Japanese positions were generally sited at creek crossings in the jungle. The advancing infantry kept strictly within range of the supporting artillery, except for short periods during the latter stages of the operation when Japanese opposition was negligible and the difficulty of moving the artillery forward quickly was too great.Report on Operations – 5 Aust Div: Operations from Sio to Saidor 20 January – 29 February 1944, Australian War Memorial: AWM54 519/6/48 A secondary outflanking movement was made inland, over the higher ground, which was usually coral cliffs covered by kunai grass and rising as high as . Since the Japanese intent was to delay rather than fight to the death, a threat to their escape route usually prompted a withdrawal. When this did not occur, the position was reduced by a combination of manoeuvre and tank, mortar, and artillery fire. During the early part of the advance, 4,700 rounds were fired in one day; but during the entire 5th Division advance from Sio to Saidor, only 30 rounds were fired. The advance was made in a series of bounds, the objective of which was usually to secure sheltered beachheads.
Allied supply was entirely by sea. Amphibian scouts from the US 532nd Engineer Boat and Shore Regiment (EBSR) of the US 2nd Engineer Special Brigade, wearing Australian uniforms,Casey, Amphibian Engineer Operations, p. 97 advanced with the infantry and reconnoitred beaches from the landward side as they were secured. If a beach looked suitable, a second reconnaissance was made from the sea. Once a beach was selected, a shore party was brought in by landing craft to set up an administrative area. Engineers improved the coastal track to enable supplies to units moving along it to be brought up by jeeps but supplies for units moving inland over the high ground had to be brought by native carriers. Artillery guns were brought forward over the tracks or else were moved by Landing Craft Mechanised (LCMs) of the 532nd EBSR. Tanks normally moved along the tracks but used LCMs to bypass obstacles. As the advance continued, new beachheads were opened up while rearward ones were closed. The advance was halted on occasion to allow for the guns to be brought forward or sufficient supplies to be accumulated at the forward beachhead.
The major problem was the weather. The monsoon caused rough seas that precluded the use of the small Landing Craft, Vehicle, Personnel (LCVPs) and restricted the operations of the larger LCMs. Because of the extremely rough seas—the most difficult that the 532nd EBSR had ever encountered—most supply missions were by night when tidal conditions were most favourable.Monthly Historical Report of Operations – 2d ESB, January 1944, U.S. Army Corps of Engineers: X-78 E-20-1 Wootten insisted that at least seven days' supplies be available in forward areas in case the weather prevented the LCMs from running. II Corps made available two trawlers, manned by the 1st Water Transport Group, to deliver rations. The Australian Army also moved supplies by DUKWs.
Battle
Fortification Point
Wootten designated Brigadier C. R. V. Edgar's 4th Infantry Brigade, a Militia formation, for the initial phase of the coastal advance, reserving his veteran Australian Imperial Force brigades for the Battle of Wareo. The brigade consisted of the 22nd and 29th/46th Infantry Battalions from Victoria and the 37th/52nd Infantry Battalion from Tasmania.Coates, Bravery Above Blunder, p. 241 Each battalion was allotted a team of advisors from the 9th Division. Under Edgar's command was C Squadron, 1st Tank Battalion, with seven Matilda tanks, 9th Platoon, C Company, Papuan Infantry Battalion, and detachments from the 532nd EBSR, Australian New Guinea Administrative Unit (ANGAU), Australian Army Service Corps (AASC) and Australian Army Medical Corps (AAMC). In support were the sappers of the 2/7th Field Company and the 24 25-pounders of the 2/6th Field Regiment. In case Edgar got into trouble, the 20th Infantry Brigade was in reserve, on six hours notice.