two sides met again. Japanese torpedoes shattered the American formation, but the Japanese did not follow up their advantage, with one destroyer sunk from each side. On November 2, the Japanese committed two heavy cruisers, two light cruisers, and six destroyers to attack the American beachhead on Bougainville Island. In another night action, this time at Empress Augusta Bay, an American force of four light cruisers and eight destroyers intercepted the Japanese and defeated them, sinking a light cruiser and a destroyer. Americans suffered no losses, with a single destroyer damaged. The Japanese had lost their tactical advantage in night engagements. Adding to their predicament was the increasing Allied strength in the region, demonstrated when the Second Fleet arrived at Rabaul on November 5 with six heavy cruisers to engage American naval forces off Bougainville; they were immediately subjected to an attack by carrier aircraft. Four of the cruisers were damaged and forced to return to Japan for repairs and the operation ended as a complete fiasco. This marked the end of major IJN operations in the South Pacific and the end of Rabaul as a major base. The conclusion that the IJN had lost its edge in night combat was confirmed later in November at Cape St George, when a force of American destroyers intercepted five Japanese destroyers, sinking three of them at no loss. Collapse of the defensive perimeter (1943-44) Admiral Isoroku Yamamoto had been killed on April 18, 1943. The following day, Admiral Mineichi Koga succeeded Yamamoto as Commander-in-Chief of the Combined Fleet. In May 1943, the Japanese prepared the Operation Z or the Z plan, which envisioned the use of the IJN to counter American forces threatening the Japanese outer defense perimeter line. This line extended from the Aleutians down through Wake, the Marshall and Gilbert Islands, Nauru, the Bismarck Archipelago, New Guinea, then westward past Java and Sumatra to Burma. In 1943–44, Allied forces in the Solomons began driving relentlessly to Rabaul, eventually encircling and neutralizing the stronghold. With their position in the Solomons disintegrating, the Japanese modified the Z Plan by eliminating the Gilbert and Marshall Islands, and the Bismarcks as vital areas to be defended. They then based their possible actions on the defense of an inner perimeter, which included the Marianas, Palau, Western New Guinea, and the Dutch East Indies. Meanwhile, in the Central Pacific a major American offensive was initiated, beginning