Resting on pillows of morning air, a lone eagle stares at the ancient road of many-citied Syria. There's something strange below, beyond understanding, too big to eat. An exotic creature glistens and crawls in the early summer sun, like a gigantic bronze-clad caterpillar. With forty thousand mouths to feed, it gobbles up crops and herds, leaving little more than blight in its path. Local people are gaping in stunned apprehension; many scuttle into their farm houses to hide. The dreaded Roman army's on the march, in a massive troop movement that'll change the course of history.

Its head is a man, charming and well spoken, but notorious for sordid greed. His love of lucre could make Midas seem lacking in monetary spirit, or Croesus neglectful of wealth. Former triumvir and the richest man in Rome, Marcus Licinius Crassus is in the Roman province to launch an invasion of the affluent Parthian empire next door to the east. Through wealth and political manoeuvring he's procured the command of seven legions. It's the greatest success of his career, but only the penultimate step. Much more than this, even more than the expected spoils of war, are at stake. He's burning to become the number one citizen of Rome, civis princeps, never stops thinking about it. For that he must command an army that wins a glorious victory – on a par with what Scipio and his rival Pompey, not to mention the great Caesar, have achieved. Parthia is the place to do it, the successor to the Persian Empire of the Achaemids.

At huge expense, he's paid for the army, bought his post in effect; so he owns it like a chattel; but he can't admit to thinking like that. He doesn't really own it; that's too outrageous a claim even for him to make. Private armies went out with the cruel Marius a long time ago. Anyway, it's a powerful instrument and he has the right to use it, a risky benefit admittedly for a man who has limited, albeit not negligible, experience as a commander.

He's prone to congratulate himself on being clever, exceptionally so, and much more focussed than the average successful man. Deep down though, he knows his real skill is in amassing wealth, using astute and often unconscionable means to do it. He's sensitive about this to the point of denial, not because of the dubious morality implicit, which is common anyway in Rome, but because he wants to tread the road to glory, a sublime path reserved exclusively to great military leaders, not plutocrats.

He feels the hot flush of glory already; why shouldn't he? He's in charge of the magnificent machine that brought glory to Pompey, the hero who must be upstaged. It can work wonders, as everybody knows. Prowess earned from the harsh discipline and novel tactical skills which moulded it provokes a dark shudder whenever it's on display abroad. There's nothing like it, never has been. Its unified and ordered structure, so different from the emotional rabble of other armies, forms an organic whole, a terrible colossus of preternatural power. He has it now; he alone can bend it to his will. With it he can satisfy that longing which drains all pleasure from his life, which stings his ego everyday with the pain that he's not number one, but could be, deserves to be, must be.

Soon he'll organise the Parthian army in a catacylysmic battle that'll pit West against East and decide the balance for years to come. He can't wait for it to begin.

The long serpentine line of might, of polished breastplate and helmet flashing in the harsh Asian light, dazzles the onlookers lining the road. It's like the time Apollo arrived in his guise of the sun at the ambrosial feast on Mount Olympus and stunned the gods into silence. The intruders radiate a self confidence that cowers all, not caring that it strays perilously close to the line that separates pride from hubris.

The road they're on is unique. It's by far the longest overland trading route in the world – stretching through Syria across Parthia and the Caspian Sea into the Central Asian steppes and man-eating deserts. It goes beyond the great mountain barrier which keeps hidden the strange people on the other side. Travellers tell stories of how eastern sands hide rich kingdoms of strange barbarians, and how it runs through them – a thoroughfare of mystery and romance which only wild imaginings can sense from this far west.

Like the sun and the moon it has a spirit, a personality – at times genial, at others cruel. The things that happen along it, often astonishing to the most jaded observer, seem to be steered by an invisible hand, yet it can be as unpredictable as the gods. In the Roman Empire it's simply referred to as the Caravan Road, and the safest way to travel on it is in convoys of long camel trains, for wild brigands constantly break its peace.

Today the great trade connector foresees that it, itself, will play a vital role in the curious chain of events soon to take place, happenings which are destined to resonate for centuries to come in the most unexpected section of its long pathway. Sometimes the part it will play in them will gladden its spirit and sometimes sadden it, but, happy or sad, the forces it facilitates will change the world.

In the first legion is a centurion with a curious habit. He's an avid reader of the Greek classics, brings them on campaign. His comrades josh him about it, but not too much; underneath they see him as down to earth really. They know they can rely on him in a difficult situation. He can be found late at night reading by the shaky flame of an oil lamp, sometimes crouching over an unrolled parchment of Plato's Phaedra, or Aristotle's Metaphysics, at other times the sayings of Zeno and the Stoics, or poetry. He knows the first part of the Odyssey by heart.

He's not a bookish type. Quite the contrary, although he finds the tomes interesting – especially the engrossing stories of spectacular deeds, of tragic flaws in great men, of uncompromising morality, of building strength in character, of the erratic role fate plays in the lives of men. Nevertheless, he admits the pleasure of reading them is not his main motive. His mother taught him that knowledge of the Classics is the key to social advancement – a talisman to influencing people of stature.

Quotations from them buttress arguments, giving the speaker an aura of authority. A shallow reason perhaps, but he doesn't care; if it works, he's for it. Loot and plunder and the excitement of action are more important drivers, and above all, personal ambition. He is after all a soldier in an elite military force, not a school teacher or philosopher. Not withstanding this, he can't help allowing some of the meaning of the literature to filter through his hardened exterior, sometimes to his discomfort, for often it contains wisdom that doesn't accommodate his compulsive desire to get ahead in the world.

In the camp outside a town whose name is not worth remembering, he's reading Plato's Republic – the part containing the allegory of the prisoners in the cave. Chained their entire lives facing a wall inside a cave, they see shadows cast by a fire outside of people carrying bundles. To them the shadows are reality for that is all they've ever seen. When they're released, they encounter the actual figures but refuse to believe they're real and that they've been living under an illusion. It takes a painful transition before they're disabused.

In the middle of reading it for the second time and...