Battle of Sio 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 . Following the defeat of the Japanese in the Battle of Sattelberg , Australian Army forces broke through the Japanese positions around Finschhafen . Constant pressure from US 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 , employing 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 . However , the Allies failed to seize the opportunity to completely destroy the Japanese forces . During the advance , Australian troops captured Japanese cryptographic materials . This would have 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 had been previously possible . 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 . The initiative passed to Major General Hatazō Adachi ' s Japanese Eighteenth Army which launched a series of counter - attacks against Major General George Wootten ' s 9th Division in the Battle of Finschhafen . At the Battle of Sattelberg , Wootten finally inflicted a crushing defeat on Adachi . The Japanese were defeated , but they 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 . Lieutenant General Frank Berryman , the commander of Australian II Corps , now urged Wootten to commence a coastal advance that would 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 . 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 finally 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 . 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 : 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 . When General Berryman saw Vice Admiral Thomas C . Kinkaid on 14 December , he made a point of congratulating him on the work his PT boats were doing . 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 . 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 . A secondary outflanking movement would be 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 would usually prompt 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 , some 4 , 700 rounds were fired in just 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 , 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 jeep s 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 . 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 DUKW s . 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 . 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 tank s , 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 - pounder s of the 2 / 6th Field Regiment . In case Edgar got into trouble , the 20th Infantry Brigade was in reserve , on six hours notice . To support the advance , a beach maintenance area was prepared at a beach at the mouth of the Kalueng River , which involved removing underwater and landward obstacles . So that the tanks and jeeps could immediately support the advance , a bridge was required over the Kalueng River . A preliminary operation by the 22nd Infantry Battalion on 3 December 1943 secured a crossing area and a log bridge was constructed . The operation jumped off on 5 December , with the 29th / 46th Infantry Battalion passing through the bridgehead established by the 22nd . It soon came under sporadic enemy fire , and a tank was disabled by a land mine . In the face of mounting opposition , the advance was halted near the lagoon . It resumed the next day , and the Japanese withdrew after an artillery bombardment , their orders being " while avoiding any decisive engagement " to " carry out successful resistance to try to delay the enemy advance " . This became the pattern , with the Japanese preferring to withdraw rather than suffer heavy casualties . On 10 December , Edgar brought all three of his battalions into the line and by 14 December they were closing in on Lakona , a key position on the 20th Division ' s retreat route . The tanks had difficulty keeping up with the advance . One obstacle was mines . On the first day , the 2 / 7th Field Company lifted fourteen mines but a tractor broke a track running over a mine . On 7 December , two more tanks were damaged by mines , one beyond repair . The sappers then cut a new track which they corduroyed . To support the attack on Lakona , the tanks had to cross a creek with steep coral banks , swollen by a torrential downpour . While efforts were made to bring up a compressor , tanks fired into banks and holes were packed with explosives and detonated . On the evening of 16 December , the infantry were joined by five Matilda tanks , and the Japanese positions were overrun . Some 47 Japanese dead were counted ; another 17 were killed in mopping - up operations the next day . On 20 December , aided by four Matilda tanks and 750 rounds from the 25 - pounders , the 4th Infantry Brigade occupied the Fortification Point area . Between 5 and 20 December , the 4th Infantry Brigade reported 65 killed and 136 wounded . Japanese casualties were 420 killed and 136 found dead , victims of disease , malnutrition and suicide . Only six Japanese prisoners were taken . Sio . On 21 December 1943 , the 20th Infantry Brigade passed through the 4th Infantry Brigade and initiated the second stage of the pursuit . Generals Blamey , Berryman and Wootten visited the acting brigade commander , Lieutenant Colonel N . W . Simpson , and emphasised that the brigade was to minimise casualties where possible by employing artillery and tanks . That afternoon , the lead company of the 2 / 13th Infantry Battalion reached Hubika . The battalion diarist recorded : A halt was called for Christmas while supplies were brought up . Units had a rest day . Most dined on turkey , ham , roast potatoes and Christmas pudding , and Christmas services were held . General Blamey insisted that Christmas fare be available to all units , and extraordinary efforts were made to carry out his directive . In one instance , a Piper Cub of No . 4 Squadron RAAF dropped Christmas fare to a Papuan Company on a long range patrol . The advance resumed on 27 December , preceded by air strikes by 18 B - 25 Mitchell and 12 Boston bombers . The 2 / 15th Infantry Battalion and tanks of A Squadron , 1st Tank Battalion took the lead on 31 December ; they reached Sialum on 2 January 1944 . This had a sheltered beach which served as a maintenance area . That day , the American landing at Saidor placed a large force across the Japanese escape route . Before moving over the mountains , the 20th Division headquarters elected to destroy its cryptographic materials rather than carry them . As the wet weather made burning them page by page a slow and difficult process , and a fire might attract the attention of the Allied Air Forces , someone decided to simply bury them in a steel trunk in a stream bed . An Australian sapper checking the stream bed for booby traps with a metal detector discovered it , and it was dug up in the belief that it was a mine . An intelligence officer recognised the contents as code books and soon it was on its way to the Central Bureau in Brisbane . There , the pages were carefully dried out and photographed . On 4 February 1944 , Central Bureau codebreakers decrypted a thirteen - part message that laid out the decisions reached at a conference of high ranking Japanese officers . Copies of the material were quickly sent to Arlington Hall . In January 1944 , Arlington Hall had decrypted 1 , 846 Japanese army messages . In March 1944 , with the Sio code books in hand , it decrypted 36 , 000 messages . On 11 January a platoon of the 2 / 17th Infantry Battalion replaced a rope ladder , and after climbing it and two wooden ladders reached an area atop a cliff which had once been a Japanese headquarters . It would have been a formidable position if defended , but it was not . The main body of the battalion followed the next day . The Goaling River was crossed in small boats left behind by the Japanese on 13 January and it entered Nambariwa , where one prisoner was taken , six Japanese were shot , and nine found dead . On 15 January , Sio was taken . The Sio - Nambariwa area was found to have been the principal Japanese supply area , and a large number of fuel , supply , and stores dumps were found . During the advance from Fortification Point to Sio , 303 Japanese had been killed or found dead , and 22 captured . The 20th Infantry Brigade had lost 3 killed and 13 wounded , but 958 had been evacuated sick , mostly with malaria , and an epidemic of dengue had also taken its toll . Large quantities of Japanese equipment had been captured , including six 75 mm guns , three 37 mm guns and three 20 mm guns . Saidor . At 1800 on 20 January 1944 , the headquarters of Major General A . H . Ramsay ' s 5th Division , which had come up from Lae , replaced that of the 9th Division . At the same time the 8th Infantry Brigade replaced the 20th . The 8th Infantry Brigade , which had spent much of the war on garrison duty in Western Australia , began departing Cairns on 10 January . The original intention had been for it to go to Lae to relieve the 29th Infantry Brigade , which had fought in the Salamaua - Lae campaign , but in December it was decided to ship it directly to Finschhafen . The brigade contained three infantry battalions , the 4th , 30th and 35th , all from New South Wales . It also still had the support of the 2 / 12th Field Regiment , 532nd EBSR , and A Company , Papuan Infantry Battalion . The brigade suffered its first casualties on the night of 21 / 22 January in a friendly fire incident , which was a common occurrence with units inexperienced in jungle warfare . Two Australians were killed and two wounded by their own comrades . On 22 January a native reported seeing seven Japanese in the hills south west of Sio and a patrol was sent out under Corporal Bengari to investigate . On arriving in the vicinity on 24 January , a local reported that another 22 Japanese had arrived . The next morning , Bengari and his five companions ambushed the Japanese and killed them all before they could fire a shot . Wirraway and Boomerang aircraft of No . 4 Squadron RAAF scouted ahead of the advance . Its aerial reconnaissance work let the Australians and Papuans know where opposition could be expected , thereby speeding up the advance . The pilots noted Japanese parachutes , signs that the Japanese were receiving supplies by air . On 4 February , the Australians were also forced onto air supply , as swollen rivers washed out a number of bridges . Each day the Papuans killed 12 to 15 Japanese , but not until 8 February was the Japanese rearguard encountered at Weber Point and a formal attack made . Five Japanese were killed . In all , 53 Japanese were killed and four captured that day . Two Australians were wounded . The next day another 61 Japanese were killed and 9 captured , this time without any Australian casualties . On 10 February , the 30th Infantry Battalion encountered two American soldiers at Yagomai , thereby linking up with the American force at Saidor . The 8th Infantry Brigade now began to mop up the area . On 18 February , the 35th Infantry Battalion attacked a Japanese force near Gabutamon , killing 40 . Finding a force of about 100 Japanese at nearby Tapen , they attacked , killing another 52 Japanese for the loss of one man wounded , while the Papuans on their flanks killed another 51 , of whom 43 were accounted for by Corporal Bengari and two other Papuans . The next day , the Papuans found and killed another 39 Japanese in the vicinity . At Tapen , the Australians and Papuans also found evidence that the Japanese had resorted to cannibalism . In the period of 20 January – 1 March 1944 , 734 Japanese were killed , 1 , 775 found dead and 48 were captured . Australian casualties came to four killed and six wounded . Aftermath . Both sides managed to accomplish their objectives ; the Japanese managed to withdraw , while the Australians exacted a terrible toll . The balance of losses was overwhelmingly against the Japanese , both in terms of men and equipment . It seems that only about 4 , 300 of the 7 , 000 troops under the command of the Japanese 20th Division that had originally been forward of Sio survived the withdrawal , and many of them were rendered ineffective through wounds , sickness , malnutrition and exhaustion . However , the opportunity to destroy the Japanese 51st Division was not seized . These troops would live to fight the Americans at the Battle of Driniumor River later in the year , and the Australians in the Aitape – Wewak campaign in 1945 . On the other hand , the new base at Finschhafen was no longer threatened by the Japanese , and would become an important staging point for the Western New Guinea campaign . The capture of the Japanese ciphers at Sio would allow General MacArthur to carry out Operations Reckless and Persecution with a plan based upon sound intelligence rather than just his own intuition .