 Complete the following phrase:  The Japanese had captured Bougainville in early 1942, and had subsequently built it into a significant base with large numbers of aircraft and troops, in order to protect the fortress at Rabaul. Following this, Allied planners determined that their key strategy in the region would be reduction of the base at Rabaul. To do this, they needed airfields within range, and as such, it was decided to secure the western part of Bougainville, 