 the final Australian advance began , bringing the campaign to an end .   = = Background = =   = = = Strategic situation = = =   Japanese forces had landed on Bougainville in early 1942 , capturing it from the small force of Australians garrisoning the island . They had subsequently developed several airbases on the island , using it to conduct operations in the northern Solomon Islands and to attack the Allied lines of communication between the United States , Australia and the Southwest Pacific Area . These bases also helped protect Rabaul , the major Japanese garrison and naval base in Papua New Guinea , and throughout 1943 , Allied planners determined that Bougainville was vital for neutralising the Japanese base around Rabaul .   US Marines conducted an amphibious landing at Cape Torokina , on the western coast of the island , north of Empress Augusta Bay , in November 1943 . After an initial counter @-@ attack , the US Marines had been replaced by a garrison of US Army troops who began consolidating their position around Torokina , establishing a strong perimeter . In March 1944 , the Japanese launched a heavy counter @-@ attack , which was turned back with heavy casualties . After this , the situation on Bougainville became largely static , as the Japanese focused primarily on subsistence , and the US forces chose to adopt a mainly defensive posture focused on maintaining the perimeter around Torokina .   In late 1944 , as part of plans to free US troops up for the Philippines campaign , the Australian II Corps — consisting of mainly Militia troops under the command of Lieutenant General Stanley Savige — took over responsibility for Allied operations on Bougainville from the American XIV Corps . Australian forces began arriving on the island between November and December 1944 , initially establishing themselves around the US base at Torokina . Due to inaccurate intelligence , Savige mistakenly believed that the Japanese forces on the island numbered just 17 @,@ 500 men , and he consequently decided that the Australians would pursue an aggressive campaign to clear the Japanese from Bougainville in order to free their troops for subsequent operations elsewhere , rather than maintaining the defensive posture the US forces had adopted . However , Allied estimates of Japanese strength were later found to be grossly inaccurate and after the war it was found that the number of Japanese on the island at this time was closer to 40 @,@ 000 .   The campaign that Australian planners developed entailed three separate drives : in the north , it was planned that Japanese forces would be forced into the narrow Bonis Peninsula and contained ; in the centre the seizure of Pearl Ridge would give the Australians control of the east – west avenues of approach , as well as affording them protection against further counter @-@ attacks , while also opening the way for a drive to the east coast ; and the main campaign in the south , where the bulk of the Japanese forces were concentrated around Buin .   = = = Preliminary moves = = =   Following the capture of Pearl Ridge in December 1944 , the Australian 7th Brigade had been moved south and allocated to the drive towards Buin . In late March and early April 1945 , they had fought the Battle of Slater 's Knoll after which a brief lull followed as the Australians paused to shorten their supply lines . Meanwhile , the survivors of the Japanese force , heavily demoralised by their defeat , withdrew towards the Hongorai River . The Australian 3rd Division was then ordered to resume its advance south , being tasked with capturing the Hari River , while the Hongorai was also included as an " intermediate objective " . Japanese strength in the southern sector was estimated by the Australians at about 10 @,@ 500 men , of which 2 @,@ 300 were believed to be directly opposing the 3rd Division . The 15th Brigade was considered to be the most experienced of the Australian units on Bougainville at the time and was moved up to relieve the 7th Brigade , which was in need of rest .   Under the command of Brigadier Heathcote Hammer the 15th Brigade consisted of three infantry battalions as well as two troops of tanks from the 2 / 4th Armoured Regiment , engineers from the 15th Field Company , a battery of 155 @-@ mm guns from ' U ' Heavy Battery , field artillery from the 2nd Field Regiment , and a number of smaller support units . The 58th / 59th Infantry Battalion took over responsibility for Slater 's Knoll replacing the 25th Infantry Battalion while the 24th Infantry Battalion took up a position across the Buin Road . The brigade 's third battalion — the 57th / 60th Infantry Battalion — did not join them until the beginning of May , and so the 7th Brigade 's 9th Infantry Battalion continued patrolling operations north of the Huio River , in the Rumiki area , until the 57th / 60th could dispatch elements to relieve them . Further inland , the 2 / 8th Commando Squadron advanced in a wide arc to the south @-@ east , defending the brigade 's left , or eastern , flank .   The Japanese forces opposing the Australians belonged to the
Ġthe Ġfinal ĠAustralian Ġadvance Ġbegan Ġ, Ġbringing Ġthe Ġcampaign Ġto Ġan Ġend Ġ. Ġ Ġ Ġ= Ġ= ĠBackground Ġ= Ġ= Ġ Ġ Ġ= Ġ= Ġ= ĠStrategic Ġsituation Ġ= Ġ= Ġ= Ġ Ġ ĠJapanese Ġforces Ġhad Ġlanded Ġon ĠBou gain ville Ġin Ġearly Ġ1942 Ġ, Ġcapturing Ġit Ġfrom Ġthe Ġsmall Ġforce Ġof ĠAustralians Ġgarrison ing Ġthe Ġisland Ġ. ĠThey Ġhad Ġsubsequently Ġdeveloped Ġseveral Ġair b ases Ġon Ġthe Ġisland Ġ, Ġusing Ġit Ġto Ġconduct Ġoperations Ġin Ġthe Ġnorthern ĠSolomon ĠIslands Ġand Ġto Ġattack Ġthe ĠAllied Ġlines Ġof Ġcommunication Ġbetween Ġthe ĠUnited ĠStates Ġ, ĠAustralia Ġand Ġthe ĠSouthwest ĠPacific ĠArea Ġ. ĠThese Ġbases Ġalso Ġhelped Ġprotect ĠRab aul Ġ, Ġthe Ġmajor ĠJapanese Ġgarrison Ġand Ġnaval Ġbase Ġin ĠPapua ĠNew ĠGuinea Ġ, Ġand Ġthroughout Ġ1943 Ġ, ĠAllied Ġplanners Ġdetermined Ġthat ĠBou gain ville Ġwas Ġvital Ġfor Ġneutral ising Ġthe ĠJapanese Ġbase Ġaround ĠRab aul Ġ. Ġ Ġ ĠUS ĠMarines Ġconducted Ġan Ġamphib ious Ġlanding Ġat ĠCape ĠTor ok ina Ġ, Ġon Ġthe Ġwestern Ġcoast Ġof Ġthe Ġisland Ġ, Ġnorth Ġof ĠEmpress ĠAugusta ĠBay Ġ, Ġin ĠNovember Ġ1943 Ġ. ĠAfter Ġan Ġinitial Ġcounter Ġ@ - @ Ġattack Ġ, Ġthe ĠUS ĠMarines Ġhad Ġbeen Ġreplaced Ġby Ġa Ġgarrison Ġof ĠUS ĠArmy Ġtroops Ġwho Ġbegan Ġconsolid ating Ġtheir Ġposition Ġaround ĠTor ok ina Ġ, Ġestablishing Ġa Ġstrong Ġperimeter Ġ. ĠIn ĠMarch Ġ1944 Ġ, Ġthe ĠJapanese Ġlaunched Ġa Ġheavy Ġcounter Ġ@ - @ Ġattack Ġ, Ġwhich Ġwas Ġturned Ġback Ġwith Ġheavy Ġcasualties Ġ. ĠAfter Ġthis Ġ, Ġthe Ġsituation Ġon ĠBou gain ville Ġbecame Ġlargely Ġstatic Ġ, Ġas Ġthe ĠJapanese Ġfocused Ġprimarily Ġon Ġsubsistence Ġ, Ġand Ġthe ĠUS Ġforces Ġchose Ġto Ġadopt Ġa Ġmainly Ġdefensive Ġposture Ġfocused Ġon Ġmaintaining Ġthe Ġperimeter Ġaround ĠTor ok ina Ġ. Ġ Ġ ĠIn Ġlate Ġ1944 Ġ, Ġas Ġpart Ġof Ġplans Ġto Ġfree ĠUS Ġtroops Ġup Ġfor Ġthe ĠPhilippines Ġcampaign Ġ, Ġthe ĠAustralian ĠII ĠCorps ĠâĢĶ Ġconsisting Ġof Ġmainly ĠMilitia Ġtroops Ġunder Ġthe Ġcommand Ġof ĠLieutenant ĠGeneral ĠStanley ĠSav ige ĠâĢĶ Ġtook Ġover Ġresponsibility Ġfor ĠAllied Ġoperations Ġon ĠBou gain ville Ġfrom Ġthe ĠAmerican ĠXIV ĠCorps Ġ. ĠAustralian Ġforces Ġbegan Ġarriving Ġon Ġthe Ġisland Ġbetween ĠNovember Ġand ĠDecember Ġ1944 Ġ, Ġinitially Ġestablishing Ġthemselves Ġaround Ġthe ĠUS Ġbase Ġat ĠTor ok ina Ġ. ĠDue Ġto Ġinaccurate Ġintelligence Ġ, ĠSav ige Ġmistakenly Ġbelieved Ġthat Ġthe ĠJapanese Ġforces Ġon Ġthe Ġisland Ġnumbered Ġjust Ġ17 Ġ@ , @ Ġ500 Ġmen Ġ, Ġand Ġhe Ġconsequently Ġdecided Ġthat Ġthe ĠAustralians Ġwould Ġpursue Ġan Ġaggressive Ġcampaign Ġto Ġclear Ġthe ĠJapanese Ġfrom ĠBou gain ville Ġin Ġorder Ġto Ġfree Ġtheir Ġtroops Ġfor Ġsubsequent Ġoperations Ġelsewhere Ġ, Ġrather Ġthan Ġmaintaining Ġthe Ġdefensive Ġposture Ġthe ĠUS Ġforces Ġhad Ġadopted Ġ. ĠHowever Ġ, ĠAllied Ġestimates Ġof ĠJapanese Ġstrength Ġwere Ġlater Ġfound Ġto Ġbe Ġgrossly Ġinaccurate Ġand Ġafter Ġthe Ġwar Ġit Ġwas Ġfound Ġthat Ġthe Ġnumber Ġof ĠJapanese Ġon Ġthe Ġisland Ġat Ġthis Ġtime Ġwas Ġcloser Ġto Ġ40 Ġ@ , @ Ġ000 Ġ. Ġ Ġ ĠThe Ġcampaign Ġthat ĠAustralian Ġplanners Ġdeveloped Ġent ailed Ġthree Ġseparate Ġdrives Ġ: Ġin Ġthe Ġnorth Ġ, Ġit Ġwas Ġplanned Ġthat ĠJapanese Ġforces Ġwould Ġbe Ġforced Ġinto Ġthe Ġnarrow ĠBon is ĠPeninsula Ġand Ġcontained Ġ; Ġin Ġthe Ġcentre Ġthe Ġseizure Ġof ĠPearl ĠRidge Ġwould Ġgive Ġthe ĠAustralians Ġcontrol Ġof Ġthe Ġeast ĠâĢĵ Ġwest Ġavenues Ġof Ġapproach Ġ, Ġas Ġwell Ġas Ġaff ording Ġthem Ġprotection Ġagainst Ġfurther Ġcounter Ġ@ - @ Ġattacks Ġ, Ġwhile Ġalso Ġopening Ġthe Ġway Ġfor Ġa Ġdrive Ġto Ġthe Ġeast Ġcoast Ġ; Ġand Ġthe Ġmain Ġcampaign Ġin Ġthe Ġsouth Ġ, Ġwhere Ġthe Ġbulk Ġof Ġthe ĠJapanese Ġforces Ġwere Ġconcentrated Ġaround ĠBu in Ġ. Ġ Ġ Ġ= Ġ= Ġ= ĠPrel iminary Ġmoves Ġ= Ġ= Ġ= Ġ Ġ ĠFollowing Ġthe Ġcapture Ġof ĠPearl ĠRidge Ġin ĠDecember Ġ1944 Ġ, Ġthe ĠAustralian Ġ7 th ĠBrigade Ġhad Ġbeen Ġmoved Ġsouth Ġand Ġallocated Ġto Ġthe Ġdrive Ġtowards ĠBu in Ġ. ĠIn Ġlate ĠMarch Ġand Ġearly ĠApril Ġ1945 Ġ, Ġthey Ġhad Ġfought Ġthe ĠBattle Ġof ĠSlater Ġ' s ĠKn oll Ġafter Ġwhich Ġa Ġbrief Ġlull Ġfollowed Ġas Ġthe ĠAustralians Ġpaused Ġto Ġshorten Ġtheir Ġsupply Ġlines Ġ. ĠMeanwhile Ġ, Ġthe Ġsurvivors Ġof Ġthe ĠJapanese Ġforce Ġ, Ġheavily Ġdemoral ised Ġby Ġtheir Ġdefeat Ġ, Ġwithdrew Ġtowards Ġthe ĠHong or ai ĠRiver Ġ. ĠThe ĠAustralian Ġ3 rd ĠDivision Ġwas Ġthen Ġordered Ġto Ġresume Ġits Ġadvance Ġsouth Ġ, Ġbeing Ġtasked Ġwith Ġcapturing Ġthe ĠHar i ĠRiver Ġ, Ġwhile Ġthe ĠHong or ai Ġwas Ġalso Ġincluded Ġas Ġan Ġ" Ġintermediate Ġobjective Ġ" Ġ. ĠJapanese Ġstrength Ġin Ġthe Ġsouthern Ġsector Ġwas Ġestimated Ġby Ġthe ĠAustralians Ġat Ġabout Ġ10 Ġ@ , @ Ġ500 Ġmen Ġ, Ġof Ġwhich Ġ2 Ġ@ , @ Ġ300 Ġwere Ġbelieved Ġto Ġbe Ġdirectly Ġopposing Ġthe Ġ3 rd ĠDivision Ġ. ĠThe Ġ15 th ĠBrigade Ġwas Ġconsidered Ġto Ġbe Ġthe Ġmost Ġexperienced Ġof Ġthe ĠAustralian Ġunits Ġon ĠBou gain ville Ġat Ġthe Ġtime Ġand Ġwas Ġmoved Ġup Ġto Ġrelieve Ġthe Ġ7 th ĠBrigade Ġ, Ġwhich Ġwas Ġin Ġneed Ġof Ġrest Ġ. Ġ Ġ ĠUnder Ġthe Ġcommand Ġof ĠBrig adier ĠHeath c ote ĠHammer Ġthe Ġ15 th ĠBrigade Ġconsisted Ġof Ġthree Ġinfantry Ġbatt alions Ġas Ġwell Ġas Ġtwo Ġtroops Ġof Ġtanks Ġfrom Ġthe Ġ2 Ġ/ Ġ4 th ĠArm oured ĠRegiment Ġ, Ġengineers Ġfrom Ġthe Ġ15 th ĠField ĠCompany Ġ, Ġa Ġbattery Ġof Ġ155 Ġ@ - @ Ġmm Ġguns Ġfrom Ġ' ĠU Ġ' ĠHeavy ĠBattery Ġ, Ġfield Ġartillery Ġfrom Ġthe Ġ2 nd ĠField ĠRegiment Ġ, Ġand Ġa Ġnumber Ġof Ġsmaller Ġsupport Ġunits Ġ. ĠThe Ġ58 th Ġ/ Ġ59 th ĠInfantry ĠBattalion Ġtook Ġover Ġresponsibility Ġfor ĠSlater Ġ' s ĠKn oll Ġreplacing Ġthe Ġ25 th ĠInfantry ĠBattalion Ġwhile Ġthe Ġ24 th ĠInfantry ĠBattalion Ġtook Ġup Ġa Ġposition Ġacross Ġthe ĠBu in ĠRoad Ġ. ĠThe Ġbrigade Ġ' s Ġthird Ġbattalion ĠâĢĶ Ġthe Ġ57 th Ġ/ Ġ60 th ĠInfantry ĠBattalion ĠâĢĶ Ġdid Ġnot Ġjoin Ġthem Ġuntil Ġthe Ġbeginning Ġof ĠMay Ġ, Ġand Ġso Ġthe Ġ7 th ĠBrigade Ġ' s Ġ9 th ĠInfantry ĠBattalion Ġcontinued Ġpatrolling Ġoperations Ġnorth Ġof Ġthe ĠHu io ĠRiver Ġ, Ġin Ġthe ĠRum iki Ġarea Ġ, Ġuntil Ġthe Ġ57 th Ġ/ Ġ60 th Ġcould Ġdispatch Ġelements Ġto Ġrelieve Ġthem Ġ. ĠFurther Ġinland Ġ, Ġthe Ġ2 Ġ/ Ġ8 th ĠCommando ĠSquadron Ġadvanced Ġin Ġa Ġwide Ġarc Ġto Ġthe Ġsouth Ġ@ - @ Ġeast Ġ, Ġdefending Ġthe Ġbrigade Ġ' s Ġleft Ġ, Ġor Ġeastern Ġ, Ġflank Ġ. Ġ Ġ ĠThe ĠJapanese Ġforces Ġopposing Ġthe ĠAustralians Ġbelonged Ġto Ġthe