'Halt!' the legate shouted, thrusting his arm up.

The mounted escort reined in behind him, and Vespasian strained his ears to catch the sound he had heard a moment before. No longer drowned out by the heavy clumping of hoofs on the rough native track came the faint braying of British war horns from the direction of Calleva, a few miles distant. The sprawling town was the capital of the Atrebatans, one of the few tribes allied to Rome, and for a moment the legate wondered if the enemy commander, Caratacus, had made a bold strike deep into the rear of the Roman forces. If Calleva was under attack…

'Come on!'

Kicking his boot heel into the flank of his horse, Vespasian bent low and urged his mount up the slope. The escort, a dozen of his scouts from the Second Legion, pounded along after him. It was their sacred duty to protect their commander.

The track inclined diagonally up the side of a long steep ridge, beyond which it sloped down towards Calleva.

The town was being used as the forward supply depot of the Second Legion. Detached from the army, commanded by General Aulus Plautius, the Second had been ordered to defeat the Durotrigans, the last of the southern tribes still fighting for Caratacus.

Only when the Durotrigans had been destroyed would the Roman supply lines be secure enough for the legions to advance further north and west. Without adequate supplies there would be no victory for General Plautius, and the Emperor's premature celebration of the conquest of Britain would be exposed for a hollow sham to the public in Rome.

The fate of General Plautius and his legions – indeed the fate of the Emperor himself – depended on the overstretched and slender arteries that fed the legions, and which could be severed at a stroke.

Regular columns of heavy wagons trundled from the vast base camp on the estuary of the Tamesis – the river that snaked through the heart of Britain – where provisions and equipment from Gaul were landed. For the last ten days the Second Legion had been without supplies from Calleva. Vespasian had left his forces laying siege to one of the larger hillforts of the Durotrigans while he hurried back to Calleva to investigate the matter. The Second Legion was already on reduced rations, and small groups of the enemy lay in wait in the surrounding forests, ready to attack any foraging parties that dared to range too far from the main body of the legion. Unless Vespasian managed to secure food for his men soon the Second Legion would have to fall back on the depot at Calleva.

Vespasian could well imagine the anger with which General Plautius would greet news of such a setback. Aulus Plautius had been appointed by Emperor Claudius to command the Roman army whose task was to add Britain and its tribes to the Empire. Despite Plautius' victories over the barbarous tribes the previous summer, Caratacus had raised a new army and still defied Rome. He had learned much from last year's campaigning and refused to take the field against the Roman legions. Instead, he detached columns of men to attack the supply lines of the ponderous Roman war machine. With every mile General Plautius and his legions advanced, those vital supply lines became more vulnerable.

So the outcome of this year's campaign depended on whose strategy triumphed. If General Plautius succeeded in forcing the Britons to face him on the field of battle then the legions would win. If the Britons could avoid battle and starve the legions, they might weaken them enough to force the general into a perilous retreat all the way back to the coast.

As Vespasian and his escort galloped up to the crest of the ridge the blasts on the war horns became more strident. Now the soldiers could hear men shouting, the sharp clang of weapon striking weapon, and the dull thud of blows landing on shields. The long grass was silhouetted against the clear sky, and then Vespasian beheld the scene on the far side of the ridge. To the left lay Calleva, a huge sprawl of thatched roofs of mainly squalid little dwellings, ringed by an earth rampart and palisade. A thin haze of wood-smoke hung over the town. A dark gash of churned soil marked the track leading from the tall wooden tower of the gatehouse towards the Tamesis. On the track, half a mile from Calleva, only a handful of wagons remained of a supply convoy, protected by a thin screen of auxiliary troops. Around them swirled the enemy: small clusters of heavily armed warriors and lighter troops armed with slings, bows and throwing spears. They kept up a steady rain of missiles on the supply convoy and its escort. Blood flowed from the flanks of injured oxen, and the path of the convoy was dotted with bodies.

Vespasian and his men reined in as the legate briefly considered what to do. Even as he watched, a group of Durotrigans rushed the rear of the convoy and threw themselves on the auxiliaries. The commander of the convoy, clearly visible in his scarlet cloak as he stood atop the driver's bench of the first wagon, cupped his hands to bellow an order and the convoy slowly halted. The auxiliaries beat off the attackers easily enough, but their comrades at the front of the column provided a static target for the enemy and by the time the wagons were on the move again several more of the convoy's escorts lay sprawled on the ground.

'Where's the bloody garrison?' grumbled one of the scouts. 'They must have seen the convoy by now.'

Vespasian looked towards the neatly ordered lines of the fortified supply depot built on to the side of Calleva's ramparts. Tiny dark figures were scurrying between the barrack blocks, but there were no massing ranks visible. Vespasian made a mental note to give the garrison's commander a harsh bollocking the moment he reached the camp.

If he reached the camp, he reflected, for the skirmish was between his party and the gates of Calleva.

Unless the garrison made a sortie soon the convoy would be whittled down until the enemy could wipe it out in one final charge. Sensing that the decisive moment was near, the Durotrigans were edging closer to the wagons, screaming their war cries and striking their weapons against the edges of their shields to stoke up their battle frenzy.

Vespasian tore his cloak from his shoulders. Grasping the reins tightly in one hand, he drew his sword in the other and turned to his scouts.

'Form line!'

The men looked at him in surprise. Their legate intended to charge the enemy, but that was tantamount to suicide.

'Form line, damn you!' Vespasian shouted, and this time his men responded at once, fanning out on either side of the legate, making ready their long spears. As soon as the line was ready Vespasian swept his sword down.

'Let's go!'

There was no parade-ground precision in the manoeuvre. The small party of horsemen just jabbed in their heels and urged their mounts to swoop down on the enemy pell-mell. Even as blood pounded in his ears Vespasian found himself questioning the sanity of this wild charge. It would have been easy enough to bear witness to the convoy's destruction and wait until the triumphant enemy marched away from its wreckage before making for Calleva. But that would have been cowardly, and, in any case, those supplies were desperately needed. So he gritted his teeth and clenched the sword in his right hand as he made for the wagons.

Down the slope, the sound of approaching horses caused faces to turn towards them and the barrage of missiles on the convoy slackened.

'There! Over there!' Vespasian bellowed, pointing towards a loose line of slingers and archers. 'Follow me!'

The scouts swung into line with their legate and charged obliquely across the incline towards the lightly armed Durotrigans. Ahead of the horsemen the Britons were already scattering, their roars of triumph dead on their lips. Vespasian saw that the commander of the convoy had made good use of the diversion and the wagons were once more rumbling towards the safety of Calleva's ramparts. But the leader of the Durotrigans was no fool either, and a quick glance revealed to Vespasian that the heavy infantry and chariots were already moving towards the convoy to strike before their prey reached the gates. A short distance to his front, woad-stained bodies weaved madly, desperately trying to avoid the Roman horsemen. Vespasian fixed his sight on a large slinger wearing a wolfskin around his shoulders, and lowered the point of his sword. At the last moment, the Briton sensed the horse bearing down on him, looked round sharply, eyes wide with terror. Vespasian aimed his blow a short distance down from the man's neck and braced his arm for the impact, but at the last moment the slinger threw himself flat and the blade missed.