Complex sentence: Only ever held by the Emperor of Japan as commander-in-chief of the Empire, the rank was equivalent to a "generalissimo" or general of the armies (army) and lord high admiral (navy).
Split into simpler sentences: Only ever held by the Emperor of Japan as commander-in-chief of the Empire's Armed Forces; and separately, the highest ranking officer in each of the Armed Services. The rank was equivalent to a "generalissimo" or general of the armies (army) and lord high admiral (navy).