This was the US overall plan by Admiral Chester W. Nimitz’s Pacific Ocean Areas command, in line with the directive Nimitz had received from the Combined Chiefs-of-Staff Committee after the ‘Sextant’ conference in Cairo, for continued operations in the Central Pacific Area toward the Japanese home islands (1942/44).
The plan was designed to produce a terminal offensive directed not against Japan’s main bastions and the home islands themselves, but rather against the weak points in a line stretching from the Mariana islands group to China via Formosa and the Ryukyu islands group. Nimitz and his staff felt that such a scheme would avoid the inevitable bloodbaths of tackling the Japanese main forces directly, and would achieve the same result by severing the Japanese empire into two parts. Thus Japan proper in the north would be isolated with its great population but without the oil, food and raw materials it was drawing from China and the South-East Asia, and the Japanese forces in the ‘Greater East Asia Co-Prosperity Sphere’ would be cut off from reinforcement and resupply, and would therefore probably fall victim to limited Allied offensives within a short period.
For a number of reasons, many of them strategically strong, the ‘Granite’ plan was far superior to the rival ‘Reno’ plan offered by General Douglas MacArthur’s South-West Pacific Area command and based on a northward drive from New Guinea to liberate the Philippine islands and then to invade Japan.
However, in Nimitz’s and MacArthur’s meeting with President Franklin D. Roosevelt and his staff in Hawaii, MacArthur carried the day for his plan, leading to the scaling down of ‘Granite’ into the the ‘Granite II’ concept of 1943 despite its promise of lower (but certainly not low) casualties through the exploitation of Japan’s weaknesses where MacArthur’s plan triumphed over Japan’s strengths.
In the original version of ‘Granite’, the Mariana islands group was to have been invaded in November 1944, three months after the planned capture of the Palau islands group and the Truk group of the Caroline islands group, but late in January 1944 this was altered to the capture of Truk from 1 June followed by the capture of the Mariana islands group from 1 September, or alternatively the capture of the Mariana islands group from 15 June after Truk had been bypassed.
Major elements of the ‘Granite’ and ‘Reno’ plan were combined in a US Joint Chiefs-of-Staff directive of 12 March 1944, which ordered the South-West Pacific Area to isolate Rabaul, operate along the north coast of New Guinea, and plan for an invasion of Mindanao, and the Pacific Ocean Areas to bypass Truk, take the Mariana islands group from 15 June, and capture the Palau islands group from 15 September. Finally, the South-West Pacific and Pacific Ocean Areas were to combine for the capture of Luzon or Formosa from 15 February 1945.