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 in November 1943 with landings in the Gilbert Islands. The Japanese were forced to watch helplessly as their garrisons in the Gilberts and then the Marshalls were crushed. The Japanese strategy of holding overextended island garrisons was fully exposed.