Complex sentence: Chester Nimitz was to be promoted to the new rank, whic would be referred to as "Flag Admiral" for service as the Naval componet commander for the invasion of the Japanese home islands.
Split into simpler sentences: Chester Nimitz was to be promoted to the new rank, which would be referred to as "Flag Admiral". In the new rank, Nimitz was to serve as the Naval componet commander for the invasion of the Japanese home islands.