The Sword of Jupiter by Travis Starnes

An Uncertain Future in a New Past
During the test flight of a prototype faster-than-light craft, Lt. Commander Ky is flung into an ancient past that never existed. Rome has fallen, defeated by the mighty Carthaginian Empire, its survivors chased off the continent and onto what had been the remote province of Britannia. Hounded by the Carthaginians bent on conquering the known world, Rome is on the verge of destruction. Stranded and alone, except for a tactical AI implanted in his head, Ky has lost everything and must find a way to live in a world different than anything he's ever known.

For Ky to survive, he needs a new home. For the Romans to survive, they need a miracle. But can a man from the future work with people so far in his past, or is this match doomed to failure?