Brennus squinted into the blinding light. 'There!' he pointed. 'To the right of the lead cavalrymen. Must be a mile away.'

Beyond the Gaul's outstretched hand, Romulus could just make out a faint puff of smoke curling up into the haze.

Slowly the dust cloud grew larger until it was visible to all. The thunder of horses' hooves carried through the hot, still air. As soon as the senior officers had been notified, the halt was sounded. With sighs of relief, men grounded javelins and shields, waiting for orders.

'Stay put. Drink some water, but not too much!' Bassius paced up and down the cohort, encouraging his soldiers. 'The cavalry will check it out before we have to worry.'

'Nowhere to go anyway, sir. Unless it's the next sand dune?'

The anonymous comment raised a laugh from those who could hear.

'Silence in the ranks!' roared Bassius.

Responding to further trumpet calls, the cavalry nearest the enemy took off. Their fair skin, flowing hair and moustaches clearly marked them as Gauls. Some wore chain mail but many had no armour, relying instead on their speed and agility. They were not gone for long, most returning to their position while a decurion rode back to the centre of the column to report.

'What did you see?' bellowed Brennus as the officer cantered by.

Bassius glared at the indiscipline but remained silent, keen as anyone else to know what was happening.

'A few hundred Parthians,' replied the decurion dismissively.

Murmurs of excitement rippled through the cohort.

The news did not seem to alarm Crassus. Moments later, the advance sounded once more. Romulus found himself picking up pace as the marching speed perceptibly increased. Sight of the enemy had reduced the daunting prospect of desert wastes.

The group of horsemen soon came into sight, riding to within a quarter of a mile of the Roman vanguard. The Parthians pulled across their path, sitting astride short, agile ponies. Each man wore a light jerkin, decorated trousers covered by chaps and a conical leather hat. Large case-like quivers hung from the left sides of their belts. All were carrying deeply curved composite bows, similar to those of the Nabataeans.

'They're not even wearing armour,' said Brennus contemptuously.

It was hard to feel scared. If these horse archers were all the Parthians had to offer, then the huge Roman army had little to fear.

'They're just skirmishers,' observed Tarquinius. 'Here to soften us up for the cataphracts.'

It sounded ominous.

'Those bows are made with layers of wood, horn and sinew. Gives them twice the power of any other.'

Brennus' eyes narrowed. If he could send an arrow from a Gaulish bow through chain mail, what would the Parthian weapons be able to do? A shiver ran down his spine at the thought.

Tarquinius was about to continue, but Bassius came striding past, vine cane at the ready.

The Parthians sat motionless until Publius responded to the challenge. He ordered the charge. But his men had only ridden a hundred paces before the enemy turned tail and galloped away, leaving the heavier horses floundering behind. When the Gauls reined in to conserve their mounts' energy, the archers began taunting them.

Watching carefully, Publius held his men in check.

Suddenly a wave of arrows snaked into the air. Falling in a deadly shower, it knocked many Gaulish riders to the sand. Enraged, three troops immediately broke off and charged straight at the Parthians.

'Where 's their discipline? The fools think they can ride them down,' said Tarquinius. 'The Parthians are not infantry!'

Fascinated, Romulus watched as the irregular cavalry thundered towards the archers, trailing clouds of dust. Used to smashing aside opposition with ease, the Gauls roared and whooped. He could imagine how terrifying such an attack would be for foot soldiers. Lacking mounted units of its own, the Republic relied on conquered tribes like Gauls and Iberians to provide its horsemen. Carrying lances or javelins and long slashing swords, the cavalry served as a battering ram to break up enemy formations.

The ill-disciplined charge was precisely what the Parthians desired. As the Gauls closed in, they trotted off, turning gracefully in the saddle to fire at their pursuers. Swarms of arrows filled the air and Romulus gaped in amazement at their accuracy. Within moments, only thirty Gauls of the ninety who had charged were left alive. Corpses littered the ground, staining it red with blood. Dozens of riderless horses galloped about aimlessly, many bucking and kicking in pain from their wounds. The survivors reined in and fled, losing more men as they did. Sounding the recall, Publius retreated to the main column, leaving the Parthians victorious.

Not a single warrior had been killed.

'The bastards didn't even look where they were riding.' There was respect in Brennus' voice.

'I told you they weren't infantry.'

'Have you seen them before?' asked Romulus.

'Heard rumours in Armenia. They're famous for turning in the saddle and loosing. It's called the "Parthian shot".'

'Those Gauls didn't stand a chance.'

'Attacks by the archers weaken an enemy. And when they are in disarray the heavy cavalry get sent in.' Tarquinius grimaced. 'Then they repeat it.'

'Discipline!' cried Brennus. 'The Roman shield wall can take anything if the soldiers stand fast.' He thumped his shield robustly and immediately began to doubt his own words.

Tarquinius said nothing. It was unsettling.

Romulus found it nearly impossible to ignore the dead Gauls, men whose lack of restraint had got them killed. Their bodies were a grim reminder of what happened to those who disobeyed orders. Romulus hoped it would teach Crassus to conserve his cavalry. The Etruscan's veiled comments about the lack of Roman horsemen were starting to make sense and Romulus' unease grew.

High above in the azure sky, the vultures were circling.

Tarquinius studied them for a long time.

Puzzled, Romulus stared up at the broad wingtips silhouetted against the sun. Twelve vultures. No more than he might see on any other day. But when the Etruscan lowered his gaze at last, both he and Brennus noticed that he seemed very troubled.

'Were you ever wrong, Olenus?' Tarquinius said to himself. 'Twelve.'

'What did you see?' asked Romulus.

'I'm not sure,' answered Tarquinius vaguely.

It was obvious he was holding something back.

Romulus began to speak again and Brennus lifted a finger to his lips, trying to forget Ultan's prophecy. 'The man will tell us when he's ready,' he said. 'Not before.' Now that he was more than a thousand miles from Transalpine Gaul, the big man found he did not want to know if his death was imminent.

Romulus shrugged fatalistically. No point pressing the matter. The Etruscan's predictions had got them this far.

Romulus wiped the sweat off his face. 'How much longer before they stand and face us?' he said angrily. 'Why won't the bastards fight?'

Far in the distance, a line of riders danced along the horizon.

The enemy horsemen had pulled away after the abortive Gaulish attack, giving Crassus time to think. But the general would only advance and the hot mercenaries were still trudging through deep sand.

'They've gone for more arrows,' replied the Etruscan.

Brennus smiled thinly. 'Be back soon then.'

Romulus shook a fist at the Parthians. 'Come back and fight!' he roared.

'It's a simple plan, really.' Tarquinius indicated the men around them. 'They're just tiring us out.'

One day in the furnace-like heat had taken a huge toll on Crassus' army. Instead of marching in regulation close order, most cohorts had now sagged apart. The sun beat down, sapping strength from the men. Their water bags long since empty, the weaker men were beginning to sway as they walked, while others leaned on their comrades' shoulders. Figures fell out of rank to collapse on the sand. Kicked and beaten by their officers, most struggled to their feet, while some lay unnoticed and were left to die. Such poor discipline would not normally have been tolerated, but the exhausted centurions had given up shouting. It was enough that the legions were still moving forward, although under the weight of his chain mail, shield, javelins and equipment, every soldier was struggling. Except Brennus.