and the beachhead gradually expanded. On their third attempt, the Japanese successfully landed four destroyer-loads of men just beyond the eastern limit of the American beachhead before dawn on 7 November. Despite the presence of US PT boats operating out of Puruata Island, the Japanese effected this landing completely undetected by the Americans. Nevertheless, the Marines annihilated this force the next day in the Battle of Koromokina Lagoon. In conjunction with the landing forces, the Japanese 23rd Infantry Regiment, which was assigned to the 6th Division, also began attacking the US forces, with some success on 7 November, before being beaten back the following day. While escorting one of the invasion echelons to the Torokina beachhead on 9 November, Morison recounts that some of Admiral Merrill's sailors witnessed an extraordinary incident that highlighted some of the extreme cultural differences at play in the Pacific: Parts of two Marine raider battalions drove away Japanese who were blocking the Piva branch of the Numa Numa Trail in the 8–9 November Battle for Piva Trail. The Marines then selected sites in the area for two airstrips (the fighter strip at the beach was already being built). Also on 9 November, Major General Roy S. Geiger, USMC, took over command of the I Marine Amphibious Corps from General Vandegrift. Four days later, he assumed command of the entire Torokina beachhead area from Admiral Wilkinson. By this time, the Perimeter, as it was called, covered about 7,000 yards of beach front and had a circumference of about 16,000 yards. The trails to new airstrip sites had to be cleared, and General Turnage assigned this task to the 21st Marine Regiment. A Japanese ambush in the area resulted in the 13–14 November Battle of the Coconut Grove, which ended with the Marines gaining control of the point where the Numa Numa and East West Trails crossed. Throughout early November, the Japanese carried out air raids against the US forces around Torokina; however, by 17 November losses were such that the Japanese 1st Carrier Division, which had begun with 370 planes on 1 November, was withdrawn back to Truk. The US forces were thereby able to gradually expand their perimeter out to , eventually capturing two airfields with which they could subsequently launch their own attacks against Rabaul. Following this, the Japanese troops on Bougainville essentially became isolated. Late November At Rabaul, General Imamura was still convinced