Complex sentence: He also informed Clark of the whereabouts of Main Poc who had gone to Detroit for the fall and, with 120 warriors, prepared to begin hostilities the moment Great Britain and the United States went to war.
Split into simpler sentences: He also informed Clark of the whereabouts of Main Poc who was staying in Detroit for the fall. Along with 120 of his warriors, Main Poc was preparing for hostilities the moment Great Britain and the United States went to war.