Complex sentence: In Blamey's subsequent shake-up of higher command arrangements, he told told Rowell that he was to command I Corps--his first command since his three-day stint in command of a light horse squadron at Gallipoli.
Split into simpler sentences: In Blamey's subsequent shake-up of higher command arrangements, he told told Rowell that he was to command I Corps. This was Rowell's first command since his three-day stint in command of a light horse squadron at Gallipoli.