Deliberately breaking the treaty forged by Pompey some years before, the army had crossed the Euphrates a number of times the previous autumn, plundering Parthian towns in the vicinity. Crassus was creating a casus belli. By its very nature, the campaigning had not progressed more than a few miles inland. Now an altogether different prospect faced the massed lines of legionaries and mercenaries. An unknown world lay before them.

Despite the possibility of alternative routes, they were about to leave the river behind and march into the barren wastes of Mesopotamia. The prospect filled Romulus with unease, but the friends he had grown to love showed no signs of emotion. Brennus leaned forward on his longsword, dwarfing it, while the Etruscan silently contemplated the nearby eagle standard.

Remembering Tarquinius' words, Romulus breathed deeply, looking southeast towards Crassus' first objective – Seleucia, the commercial capital of the Parthian empire. With luck, all would be well.

The bucinae sounded at last, signalling forward march. Romulus felt a push in the back. Still thinking, he did not respond immediately and the man in the rank behind shoved again with his shield boss. A Roman army moved like a machine, leaving no time for contemplation.

He noticed Tarquinius looking over his shoulder at the Sixth Legion, the regular unit immediately behind the mercenaries' position. As they watched, the standard-bearer pulled his spiked pole out of the ground, preparing to lead off the first cohort. The man had only taken one step when the wooden staff slipped in his hand, allowing the silver eagle to rotate and face backwards.

Gasps of dismay filled the air and Romulus swallowed hard.

Brennus, who hated all the eagles stood for, squared his jaw.

This was the second bad omen in as many hours.

Tarquinius was smiling faintly. Luckily, most of their comrades had not seen what had happened.

Romulus took a breath of hot desert air. Stay calm, he thought.

The veteran centurion in charge of the Sixth's first cohort instantly seized the initiative. Superstition would not stop him following his orders. 'Forward march!' he bellowed. 'Now!'

Wary of punishment, the legionaries responded quickly. But muttering continued in their midst as they moved off. There was no time to ask Tarquinius about the importance of what had just happened.

Kicking up a huge cloud of dust, the soldiers picked up speed slowly. Orders rang out as centurions and optiones fussed and bothered. Men shuffled, adjusting their loads and preparing to march as each unit got under way. The mules plodded in the rear, carrying food, gold, spare equipment and assault weapons such as catapults. The enormous column stretched for more than ten miles. Any unfortunates who had been selected to guard the baggage train cursed their luck as they swallowed the mouthfuls of choking dust left hanging in the air by the legions that had marched past.

The army advanced without incident all morning. Deep sand muffled the sounds of marching feet, creaking leather and coughing men. Temperatures rose steadily as they passed by the small settlements of the Hellenic population, a people who had been living in the area for hundreds of years.

'Alexander the Great came through here,' said Tarquinius excitedly as a larger village came into view.

Full of interest, Romulus peered at the nearby mud and brick structures. 'How can you tell?'

Tarquinius pointed. 'That temple has Doric columns and statues of Greek gods. And we crossed the river where the Lion of Macedon did. It's marked on my map.'

Romulus grinned, imagining the crack hoplites who had created history. Soldiers who had been to the end of the world and back. It seemed that under Crassus, they were being given a chance to emulate the feat.

'Crassus is no Alexander,' said Tarquinius darkly. 'Far too arrogant. And he lacks real insight.'

'Even the best general can make a mistake,' Romulus argued, recalling one of Cotta's lessons. 'Alexander came to grief against the Indian elephants.'

'But Crassus has made a fatal error before the battle even starts.' The Etruscan smiled. 'It is madness not to follow a river into the desert.'

Romulus' concern about the bad omens returned with a vengeance and he turned again to Tarquinius, who shrugged eloquently.

'The campaign's outcome is still unclear. I need some wind or cloud to know more.'

Romulus looked up at the clear blue sky. The air was completely still.

Tarquinius laughed.

So did Romulus. What else could he do? There was no going back now and despite the uncertainty of their fate, excitement was coursing through his veins.

Brennus remained silent, troubled by guilty memories of his wife and child, of Conall and Brac. If he was to die in this burning hell, it was crucial for him to know that they had not died in vain. That the Allobroges had not been wiped out for nothing. That his whole life had not been wasted.

Terraced fields filled the landscape, irrigated by channels from the Euphrates. Peasants working in the crops stared fearfully at the host. Few dared wave or speak. They held their breath as thirty-five thousand armed men tramped by in an enormous cloud of dust. The noise drowned out every other sound.

An army of that size meant only one thing in any language. War.

The general rode his favourite black horse in the heavily protected centre of the column. Trumpeters paced behind, ready to relay his orders. Astride a saddle richly adorned with gold filigree, Crassus rode with the easy grace of experience, feet dangling either side, using only the reins for control.

'Good day for an invasion,' said Crassus loudly. 'The gods favour us.'

A chorus of agreement echoed from his senior officers. The veteran legionaries marching either side of them carefully kept their faces blank. Nobody dared mention what had happened earlier.

Crassus glared round at his entourage. None of these lackeys will get in my way, he thought angrily. His time had finally come. After the soldiers had left, that fool of a priest had been crucified beside the dead bull, a clear warning to the remaining augurs not to make mistakes. The image of the sand-covered heart was locked away in the recesses of his mind. It had been nothing more than a slip of the hand; the storms that had sunk so many ships nothing but bad weather. Word about the eagle standard had not yet reached him.

'With Parthia defeated, the Senate will have no option but to grant you a full triumph, sir,' ventured one of his tribunes in an effort to please.

Crassus nodded happily at the glorious prospect of riding in a chariot through the streets of Rome, a laurel wreath on his head. He would finally be equal to his partners in the triumvirate. It was mere circumstance that had brought the rivals together, not friendship, and it had seemed a good idea at first. Sharing power for more than five years had not stopped them from continually vying for dominance. None had succeeded thus far, but Crassus had suffered more setbacks than the other two.

Thanks to Pompey's propaganda, his lead role in crushing the slave rebellion had been obscured, his rightful triumph downgraded to a parade on foot. Crassus had lived for years in the shadow of the other's military success. It galled him immensely. Whilst Pompey's career had been illustrious, he also had an uncanny ability to claim victories that were not his. It was really Lucullus who had defeated Mithridates and Tigranes in Asia Minor, Crassus thought bitterly. Not that fat fool Pompey. The same will not happen here in Parthia. The glory will be mine. All of it.

He began reflecting on Julius Caesar, who had also started well by subjugating Gaul and Belgica, making himself incredibly wealthy at the same time. Now it seemed Caesar's ambition knew no bounds. Crassus cursed. It had been a mistake to help the young noble with those huge loans. The usual tactic of keeping men in his power by refusing to accept repayment of owed money had backfired when Caesar had paid off his debt in typically confident style, sending a train of mules to Crassus' house not long before he had left for Asia Minor. Hundreds of leather bags carried by the pack animals had contained the entire outstanding amount, down to the last sestertius. There had been little Crassus could do other than accept. He scowled at the manner in which he had been outmanoeuvred by Caesar, a man nearly half his age. Never again.