Publius' Gauls rode beside the slowly moving column, their large horses also beginning to look tired. In stark comparison, the Nabataeans' mounts pranced along, riders chattering busily to each other.

Brennus pointed. 'Easy for them, eh?'

'You'll be glad of the Nabataeans when we're facing the main Parthian army,' said Romulus.

'I suppose. But I don't trust them,' the Gaul growled. 'Forever sniggering and laughing. Look!'

Romulus didn't like the sly glances being cast in their direction either.

'A couple of thousand Gaulish cavalry would be more use.'

'Not if they perform like those fools back there,' said Tarquinius dryly.

In an attempt to find relief from one of many blisters, Romulus hefted his yoke from one shoulder to another and narrowly missed the head of the man immediately behind.

'Watch what you're doing,' the soldier swore. 'Or you'll feel the tip of my gladius.'

Romulus ignored him. 'Why didn't we travel through Armenia?' he asked again. 'Crassus must have known that would be easier.' Tarquinius had not been slow to share his discontent when it became evident the army was not taking the longer, safer route.

'Impatience. This way to Seleucia takes only four weeks.'

'A month in this hell?' Brennus rolled his eyes. 'What about water?'

'Resen, one of my people 's ancestral cities, lies the other way,' added the Etruscan regretfully. He lowered his voice. 'And fewer men would have died in the mountains.'

Romulus noticed him glance up at the vultures and his suspicions grew further.

Tarquinius gestured at the Parthians in the distance. 'We should have been facing that lot on our terms, not theirs.'

'True,' replied the Gaul. 'Broken terrain would suit us far better.'

'Precisely.'

'It's what we did to the Romans in the first year,' mused Brennus. 'Attacked them on our own ground.'

'And now the Parthians are doing it to us,' Romulus chipped in. 'Crassus needs to start using the Nabataeans as protection.'

Brennus nodded approvingly at the observation while a dark shadow passed unseen over Tarquinius' face. His wish to travel east was being fulfilled, but it would be at far greater cost than the haruspex had first thought.

True to form, Tarquinius' words were prophetic. In the hours that followed, groups of Parthian archers rode in close, attempting to goad the Gauls into pursuit. If Publius' cavalry responded, more arrow storms rained down. If they did not, the enemy horsemen used them as target practice. Without bows, there was little the Gauls could do to retaliate and after a number of assaults, they had lost scores of men.

The Nabataeans seemed immune to temptation. Volleys of shafts were released if the Parthians came near, a tactic that worked well. Crassus finally realised this and Ariamnes was ordered to split his cavalry, placing half on each side of the army as a protective screen. The mercenaries were heartened by their allies' presence.

Slowly the army ground forward into the sandy wasteland.

But the Parthians immediately adapted the method of harassment. Groups of riders began picking areas the Nabataeans were not protecting at that exact time and their sudden charges from behind large dunes were harder to predict. Men on the outside of each rank became experts at spotting dust clouds driven up by the enemy's horses, early warning that an attack was imminent.

'Halt! Shields up!' echoed along the line throughout the afternoon. 'Form testudo!'

Despite their exhaustion, the soldiers had learned to respond fast. Each side of the Roman column would become a wall of shields, the men inside lifting theirs to form a roof, creating cover for all.

But no matter how fast they responded, fresh screams always rang out as the showers of Parthian arrows came scything down, the shafts finding gaps in the testudo and the men who'd obeyed orders too late. The enemy quickly realised that aiming both above and below the shields was even more effective. Soldiers dropped to the ground clutching throats, arms and legs. The hiss of arrows competed with shrieks of agony in a terrible crescendo.

Romulus was glad Brennus had insisted that they buy heavy legionary scuta. The Gaulish tribesmen of his cohort carried traditional elongated rectangular shields far thinner than standard army issue and it soon became evident that they were more susceptible to the enemy bows. If the Parthians came within less than fifty paces their arrows penetrated either type with ease. Further away, only the Gauls' shields were vulnerable. It was small consolation. All day the Parthians remained tantalisingly out of range of Roman pila, which were ineffective beyond thirty paces. Fortunately their assaults did not last long, as the enemy were driven off by Nabataean charges or pulled back when they had used all their shafts.

By mid-afternoon more than forty mercenaries had been killed and injured. The dead sprawled in the sand, fresh meat for the vultures above. As the army marched past, the wounded were left with a few guards. When the baggage train arrived, they were loaded into the wagons, their screams and cries adding to the general sense of fear and unease.

And the sun beat down mercilessly, an oven from which there was no escape. Crassus' army was being drained of its ability to fight.

Romulus' first taste of battlefield combat was not what he 'd expected. Cotta's lessons about armies meeting on a flat plain and lines of men clashing in shield walls were far from this. He ground his teeth as comrades continued falling to Parthian arrows. Even fights in the arena seemed easy now. There they were one on one, man to man. The tactic of wearing down an opponent was new to him. It was torture enduring attacks without being able to fight back.

Matters came to a head for Romulus when a lone Parthian archer returned after his comrades had just been driven off. Riding parallel, he began firing shafts at the irregulars from just outside javelin range. Half a dozen arrows later, five men lay dead and another had been maimed. The marching soldiers cringed behind their shields, each hoping he would not be next.

'Son of a whore!' Romulus yelled. He prepared himself to break rank, but Brennus quickly pulled him back.

'Wait!'

'I can kill him,' Romulus said, taking a deep breath. It was time to take a stand: too many of their comrades had been slain.

'He'll loose three arrows before you go ten steps!'

Romulus shook off the Gaul's hand proudly. 'I'm a man, not a boy, Brennus. I make my own decisions.'

The comment sank home more than he could know and Brennus released his grip. The lad's just like Brac, he thought.

Tarquinius did not look surprised.

Hefting the pila he had been training with for months, Romulus stepped out of formation.

'Get back into line, soldier!' yelled Bassius.

Ignoring the order, Romulus stabbed his second pilum into the sand and locked eyes with the Parthian. The archer's confidence was now so great that his horse had slowed to a walk and he smiled as Romulus drew back to throw.

Brennus held his breath but the arrogant rider did not even raise his bow in response.

'Waste of time,' said a soldier two ranks behind. 'He 's too far away.'

The centurion was about to bellow again, but paused.

With a grunt of effort, Romulus hurled the javelin. It curved upwards in a huge arc before coming down to skewer the Parthian through the chest. There was a roar of approval as the archer toppled slowly off his horse. It was an incredible throw and the mercenaries' spirits visibly lifted.

Romulus resumed his position and Brennus clapped him on the shoulder. 'Fine shot.'

He flushed with pleasure.

By late afternoon, the dreadful heat began to abate and the Parthians finally pulled away. Only fifteen miles had been covered instead of the regulation twenty, but Crassus called a halt before even more men collapsed. Despite their total exhaustion, every other soldier had to help build a marching camp.