The epic story of the imperial rivalry between two of the greatest empires of the ancient world - Parthian and Persian - and how they rose and eventually fell. The Roman empire shaped the culture of the western world against which all other great powers are compared.

Stretching from the north of Britain to the Sahara, and from the Atlantic coast to the Euphrates, it imposed peace and prosperity on an unprecedented scale. However, the exception lay in the east, where the Parthian and then Persian empires ruled over great cities and the trade routes to mysterious lands beyond.

This was the place Alexander the Great had swept through, creating a dream of glory and conquest which tantalised Greeks and Romans alike.

Caesar, Mark Antony and a long succession of emperors longed to follow in Alexander's footsteps.

All failed. Only here did the Roman empire slow down and eventually stop because it was unable to go any further. Following seven centuries of conflict that, ultimately, neither Rome nor Persia would win, The Eagle and the Lion delves into the clash, context and journeys of these entities of great power and the people caught in their wider struggle.