Scarrow, Simon. Eagles of the Empire

"AD 56. Battle-hardened veterans of the Roman army Tribune Cato and Centurion Macro are garrisoned at the eastern border, aware that their movements are constantly monitored by spies from dangerous, mysterious Parthia. But the enemy within could be the deadliest threat to the Legion ... and the Empire. There's a traitor in the ranks. Rome shows no mercy to those who betray their comrades, and the Empire. But first the guilty man must be discovered. Cato and Macro are in a race against time to expose the truth, while the powerful enemy over the border waits to exploit any weaknesses in the Legion. The traitor must die ..."--Publisher description