BY THE TIME THEY LEFT CARRHAE, the fifth day out from the Euphrates, the caravan had settled into a routine. Severus was an experienced cavalry officer. They took the first couple of miles at an easy walk, then halted for a quarter of an hour, adjusted the packs on the mules, let the horses have some water, checked their feet and tightened their girths. After that they alternated between a trot and a walk to avoid the muscles of the animals becoming fatigued by the same gait. They would rest for a few moments on the hour, longer at midday. Decimus would go down the line, testing the cinching on the mules and making sure that none of the saddles had slipped. Too far forward was easier on the rider, but bad for the mount.

They had come down through Batnae. Until they reached the plain of Carrhae, the road had run through bare ochre hills. They had met a few other caravans, and from time to time passed a black-tented Arab encampment. The tent-dwellers were half naked, their animals and children thin. The adults had watched the riders, silent and grave-eyed, too proud or apathetic to beg.

Although they had encountered no danger, the riders proceeded cautiously. While their armour and helmets were stowed with the baggage, each man had a sword on his hip, and a bow case and quiver, as well as a small round shield, slung from the horns of his saddle.

Severus had set the order of march. On the first day, Valens had suggested putting out scouts. Severus had overruled him. They were armed merchants, not a cavalry patrol. Even so, the officer ordered Hairan, who knew the country, to ride a couple of hundred paces ahead. Often the man of Hatra was out of sight in the hills, hidden by the lie of the land. Severus himself rode at the head of the column with Iudex, Aulus and Quintus. The four spare horses came next, led by Zabda. Behind him the ten mules, roped together, plodded along in single file after the bell-mare. Narses went with the mule train, under the eye of Decimus. At the rear, eating the dust kicked up by the hooves, were Valens and Clemens the armourer.

To begin with, Clemens had made it obvious that he was disinclined to speak. But, with the passing days, he had become a little less reserved.

‘What do you have against the easterners?’ Valens asked, passing over a skin of watered wine.

Clemens took a swig, and handed back the drink without answering.

‘They are soldiers of Rome, no different from us,’ Valens added.

Clemens frowned. His face was square, as if carved out of a block of stone. Lined and weather-beaten, it could have been the bust of some stern Roman senator from the days of the old republic.

‘Surely you do not really think they would betray us?’ Valens persisted.