At the end of April 1943, the Allies developed their plan for encircling Rabaul, codenamed “Operation Cartwheel.” It called for MacArthur to approach Rabaul from the southwest, through New Guinea and the southern Bismarcks, while Halsey would advance through the Solomons, forming two pincers that would close in on the Japanese base. Meanwhile, the Japanese were busy reinforcing and reorganizing their forces in the South Pacific in anticipation of an Allied offensive. General Hitoshi Imamura, headquartered at Rabaul, commanded Japan’s 17th Army in the Solomons; they were reinforced by the 18th Army, tasked with defending northern New Guinea.
In late June, the two-pronged Allied drive toward Rabaul began, both in New Guinea and the Solomons. Lae, on the northern New Guinea coast, fell in mid-September; U.S. forces then seized Saidor, opposite Cape Gloucester, on the westernmost tip of New Britain. By October 1943, Halsey’s forces were ready to attack Bougainville, the largest and westernmost island in the Solomon chain, located just 200 miles from Rabaul at the narrowest sea crossing. Reinforcements (some 37,500 men) from the Japanese 17th Army were sent to Bougainville, concentrated at Buin, near the island’s southern end, and on small islets off the shore of the main island. This left Empress Augusta Bay, to the north, open to a landing by U.S. Marines on November 1, 1943, four days after New Zealand forces captured the tiny Treasury Islands, south of Bougainville. In the ensuing battle, American ships engaged thinly dispersed Japanese defenders, sinking Japanese cruisers and a destroyer while the 5th Army Air Force bombed Japanese airstrips and supported the Marine landing.