'Bo' was the Japanese first stage of the occupation of Bougainville island in the Solomon islands group (28/30 March 1942).
Bougainville is the largest island of the Solomon islands group, with a length of about 130 miles (210 km) and width of 30 miles (50 km), and an area of about 3,800 sq miles (9840 km˛). The island is located near the north-western end of the chain of the Solomon islands Group, about 190 miles (300 km) to the east of Rabaul on New Britain. Bougainville is mountainous, dominated by the Emperor and Crown Prince ranges, with two active volcanoes. The taller of these is Mt Balbi, which peaks at 8,907 ft (2715 m). The lower slopes and coastal plains are covered in dense jungle. With an average annual rainfall of about 100 in (2.54 m), the island is wet throughout the year, but during the winter months south-east winds bring slightly drier conditions than the north-west winds of the summer months. Malaria and other tropical diseases are prevalent.
Late in 1941 the island had been an Australian mandated territory since the period shortly after Worlds War I, and there was an adequate anchorage with a small landing for the loading of copra at Buin, near the south-eastern end of the island, and a grass airstrip. A 1,400-ft (425-m) airstrip had been completed on Buka island, to the north-west of the Buka Passage, the narrow strip of water between Buka and Bougainville, which was also the Australian administrative centre. There were several native trails, mostly along the coast, but only the trail around the north-west coast of the island was usable by motor vehicles. The population was about 54,000 indigenous persons speaking about 18 different languages, but only 100 Europeans and 100 Asians, most of the latter being Chinese.
The evacuation of all European women and children was ordered on 12 December 1941, and that of the remaining Europeans on 18 December. However, many of the European residents refused to leave, this including a significant percentage of the missionaries on the island. Many of the Europeans and most of the Chinese were in fact evacuated later, and with considerably greater difficulty, by submarine. Among those who remained were Jack Read and Paul Mason, who became part of the 'Ferdinand' coast watcher organisation and transmitted vital early warnings of Japanese air raids against Henderson Field during the Guadalcanal campaign.
In 'Bo', the Japanese destroyers Mochizuki, Mutsuki and Yayoi escorted Rear Admiral Masao Kanazawa’s Bougainville Invasion Force with the ammunition ship Soya, carrying the 8th Special Base Force, and two auxiliary gunboats.
The Japanese landed in the area around the Buka Passage on 30 March 1942, seizing the airfield as a useful satellite to the bases it had taken and was enlarging at Rabaul. Occupation of the rest of the rest of the island was leisurely: Kieta on the north-east coast, for example, was not occupied until July. The Japanese initially made little effort to hunt down the coast watchers, who operated almost unhindered during the most crucial part of the Guadalcanal campaign.
The indigenous population of Bougainville was more co-operative with the Japanese than those of other parts of the Solomon islands group. Among the reasons for this was the evacuation of the European population, which proved deeply disturbing to the local population, who rioted at Kieta on 23 January and were brought under control only by the efforts of one of the German residents who had refused evacuation. The island had been a German possession before its seizure by Australian in World War I, and the influence of German missionaries and the fact that the Japanese had so easily driven out the Allies also had their effect on the attitude of the natives.
Japanese pressure on the coast watchers became steadily greater, and they were evacuated from the north-east coast in March 1943 by the US submarines Gato and Guardfish. In addition to the coast watchers, 12 women (including three nuns) and 27 children were evacuated.
The airfield at Buka was rapidly improved by the Japanese following the Allied 'Watchtower landing on Guadalcanal in August 1942, and boasted a power plant, underground fuel tanks, and a 2,995-ft (700-m) runway surfaced with a mixture of crushed coral and asphalt. By a time late in 1943 the Japanese had also completed airfields at Kahili, Ballale, Kara and Bonis. Another airfield was under construction at Kieta. Beset by rain and harassing air raids, the airfield at Kahili was barely usable by a time late in October 1943.