In May 1943, the Japanese prepared Operation Z or the Z Plan, which envisioned the use of Japanese naval power to counter American forces threatening the 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 Bismarck Archipelago 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 the Americans initiated a major offensive, 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 strategy of holding overextended island garrisons was fully exposed.