The Northern Solomons-- Bougainville
Bougainville at the top of the Slot was the last major obstacle on the road to Rabaul. Some 150 miles long and thirty miles wide, covering about 4,000 square miles, it was another piece of extremely hostile territory. It is the largest of the Solomon Islands, violin-shaped, split down its back by a mountain range which included two active volcanoes. Except for a coastal plain at its south end, it is covered in dense jungle. Buka Island lay just off the north coast, effectively part of the larger island. The Shortlands and the Treasury Islands were also strategically a part of the main island.
In the almost two years that Japan had occupied Bougainville, it had turned the island complex into a considerable fortress. First it had established a seaplane base. Then it had built four airfields; a fifth strip was located on Ballale, an island a few miles to the south. Under construction at Kieta was another strip. The fortress was garrisoned by some 60,000 men, two-thirds Army.
Fully aware of the looming major battle, the Japanese had prepared for it. Among other things, they had made a determined effort to sweep the islands clear of Allied coastwatchers and residual Australian