Brigade and the 23rd Brigade. On 6 October, the first elements of the headquarters detachment of the 3rd Division landed. By mid-November, the 7th Brigade had relieved the U.S. 129th and 145th Infantry Regiments. On 22 November, Savige formally took command of Allied operations on Bougainville from Griswold. By 12 December, the replacement of frontline American troops by Australians was complete, and with the exception of a few service troops, all American service personnel had departed by 1 February 1945. The 3rd Division and 11th Brigade, reinforced by the Fiji Infantry Regiment, were posted to Bougainville. The 23rd Brigade garrisoned the neighbouring islands. Australian offensive operations The Australians determined that Japanese forces on Bougainville, now numbering approximately 40,000, still had approximately 20 percent of their personnel in forward positions and that although understrength, were organized in combat-capable formations, including the 38th Independent Mixed Brigade and General Kanda's tough 6th Division. Savige issued his instructions on 23 December. Offensive operations would consist of three separate drives: In the north, the 11th Brigade would force the Japanese into the narrow Bonis Peninsula and destroy them. In the centre, the enemy was to be driven off Pearl Ridge, a feature from which both coasts of the 30-mile-wide island could be seen. From there, aggressive patrols could be launched to disrupt Japanese communications along the east coast. The main Australian drive would take place in the south where the bulk of the Japanese forces (Kanda's 6th Division) was located. It was to this goal that Savige assigned Bridgeford's 3rd Division. Central front The Battle of Pearl Ridge (30–31 December) revealed how far Japanese morale and stamina had fallen. The ridge was taken by a single battalion of Australians, suffering few casualties in the process. It was afterwards discovered that the position had been held by 500 defenders rather than the 80–90 that had originally been estimated. Activity in the central sector was from that point on confined to patrols along the Numa Numa Trail. Northern front Pursuant to General Savige's 31 December order to begin operations in the northwestern sector at the first opportunity, General J.R. Stevensons's 11th Brigade advanced along the coast, reaching the village of Rukussia by mid-January 1945. However, since the coastal plain was dominated by Tsimba Ridge, the Genga River could not be crossed in force until the Japanese had been dislodged from the crest of that ridge. In the