As the pallid pearl hue thickened along the eastern horizon Cato's keen eyes peered over the rim of his shield as he scanned the surrounding desert. The horse-archers were easily visible now, and as the light grew he was able to pick out ever more detail in the scattered screen of riders surrounding the two cohorts. Now Cato could see that their clothes and accoutrements were subtly different from those of the Parthians he had fought the year before. They were Palmyran troops, then.

There was a sick tremor of anxiety in his stomach as he wondered if these men might be loyalists, sent by the king to seek help from the Romans. If that was the case, Cato's thoughts raced on, then there had been a tragic mistake in the confusion of the night's encounter. The man he had wounded would be merely one of many who had been needlessly injured or killed.The dread thought passed almost as quickly as it had arisen. There was little chance of the Roman infantry's being mistaken for anything else and the horsemen had made no attempt to call off their attack. They were clearly hostile: followers of the traitor Artaxes and his Parthian allies.

As pale light spilled across the desert, the horsemen began to shoot more arrows, aiming high so that the shafts rose gracefully up, hung for an instant, and then plunged down at a steep angle on to the Romans.Although the auxiliaries and legionaries were well sheltered by their shields, the cart mules were not, and as Cato watched they were struck down, one after another, with pitiful shrill brays of shock and pain as the arrow heads whacked through their hides and punched deep into the flesh beneath. However, the enemy did not have things all their own way, Cato noted, as he saw one of the horse-archers suddenly thrown back in his saddle, his bow dropping from his fingers as a lead shot struck his head, killing him instantly.The body toppled from the saddle on to the ground in a small explosion of dust, and those Romans who saw it gave a lusty cheer.

'A fine shot!' Macro bellowed from the other end of the square. 'A denarius for that man, and any others you knock down!'

The offer of a reward had its effect as the slingers released their shots even more swiftly and the horsemen immediately shied away to a much greater range where their fire could not be so accurate. Cato noticed that the enemy's barrage slackened until there were clear intervals between each handful of arrows. Finally, as the sun rose over the horizon and cast long shadows across the desert, the enemy archers ceased their shooting altogether and retired a short distance to dismount and rest their horses as they took a quick meal from their saddlebags.

'Seems we have something of a stand-off,' Parmenion muttered.'They can't crack us and we can't get at them. Not until our cavalry is ordered forward.'

'Yes, it's about time for that.' Cato turned towards Macro and waved an arm to attract his friend's attention. As soon as Macro saw him, he gave Cato the thumbs-up. Cato pointed to the two bucinators standing just behind the Second Illyrian's standard and Macro nodded deliberately as he grasped Cato's intention. Cato turned towards his bucinators, but before he could give the order Parmenion grasped his arm.

'Sir! They're moving.'

Cato swung round and saw that the enemy riders had thrown down their rations and were hurriedly scrambling back into their saddles and snatching the bows from their cases.

'Looks like they're going to charge us after all.'

'Let 'em try it,' Centurion Parmenion growled. 'They'll not break into the square. Not in a fair fight.'

Cato smiled briefly. Parmenion clearly belonged to that element of the Roman military that held the view that archers were cowards. For his part Cato saw them as merely another means of waging war. Archers had their limitations as well as their advantages. Unfortunately, the present circumstances favoured their advantages.

'Close up!' Cato shouted. 'Front rank! Present javelins! Prepare to receive cavalry!'

Around him the auxiliaries and legionaries braced themselves with grim expressions as they stared at the enemy, still hurriedly mounting up and forming into loose bodies of men amid swirls of dust. As the riders gathered together, behind their serpent standards, Cato frowned.

'What the hell?'

Parmenion squinted over the ranks of the auxiliaries standing silently in front of the two officers. 'They're facing the other way. Why?'

Cato shook his head. This was strange. They were forming up quickly, as if to charge, but away from the two Roman cohorts. What was happening? Just then, the faint, strident blasts of a horn sounded in the mid-distance, from beyond the enemy horsemen.

'Reinforcements?' Parmenion wondered hopefully. 'Ours or theirs?'

'Not ours. We're the only body of Roman soldiers for a hundred miles around.'

More horns sounded, and then there was a reply from the men who had been attacking the two cohorts a moment earlier – a clear sharp note of defiance. And then they charged away from the Romans in a cloud of dust kicked up by the thundering hooves of their mounts. The Roman troops gazed after them in amazement. Macro hurried across the square to Cato.

'What the fuck is going on?'

'No idea, sir. Only that there's more horsemen out there. Might be more hostiles and those men have gone to join them, or, if we're lucky, someone's come to help us. Either way, we should call in our cavalry.'

'You're right. Do it now.'

'Yes, sir.' Cato turned to give the order to the bucinators carrying the large curved brass horns. They took a breath, puffed their cheeks and a moment later the signal blasted out. They repeated it twice before lowering their instruments and then all eyes turned back towards the receding wave of enemy horsemen. Thanks to the red-hued cloud of dust they had kicked up it was hard to pick out any detail and only once in a while could the dim figures be seen amid the sandy haze. But the sound of horns, and the faint clash of weapons and shouted war cries that carried back to Roman ears, told their own story.

'Who the hell is attacking them?' asked Macro.'I thought we were the only Romans out here?'

'Perhaps Longinus has sent a cavalry column out after us,' Centurion Parmenion suggested hopefully.

'Maybe,' said Cato. 'But I doubt it.'

'Then who is it, sir?'

'We'll know soon enough.'

As the three officers and their men continued to watch in silence, the distant fight raged on. Occasionally a figure would flee from the fight and burst free of the obscuring dust cloud to race off over the desert. Here and there a riderless horse emerged and trotted aimlessly away. At length the sounds of battle died away and then there was quiet, as the sun rose low in the sky and its blood-red beams streamed over the landscape.

Parmenion turned and called out,'Here come our boys!'

The Second Illyrian's four cavalry squadrons were galloping towards the two cohorts, armour glinting in the early morning light. Cato spared them a brief glance and then turned back. He took a sharp breath.

'Look!'

Macro and Parmenion faced round as they followed the direction of Cato's outstretched finger.

A rider had emerged from the slowly settling cloud of dust. He was dressed in black and the first rays of the rising sun played off the silver ornaments of his harness and coned helmet. Reining his horse in, he stopped to examine the Roman soldiers before him, still formed into a square.Then more figures resolved into sharp outlines behind him as other mounted men appeared. Still more rode out of the dust until at last Cato calculated that the man must have at least a hundred followers. They rode forward and stopped behind their leader and stared at the Romans.

'Great,' Macro muttered. 'Now what? Hostiles?'

Cato scratched his chin. 'Out here? More than likely. However, they've seen off those horse-archers. Let's hope that my enemy's enemy really is my friend.'