Jebe was sore and dry-mouthed, feeling as if there was dust in every joint. His lower back ached and he could only wonder at the enemy who were still there when he looked back. As the light increased, he saw the Arab horses were exhausted from the ride. Their pursuers were lolling in their saddles, but they had not fallen, or allowed the tumans to get too far ahead.

Jochi was proud of the Chin who rode with his people. They had suffered more than anyone and so many had drifted back that they formed the rear of the tumans. Still they went on. Less than half a mile separated the two armies and that had not changed since the darkest hours.

As the sun rose in glory, Khalifa passed orders down to his senior men. He had suffered through the night, with cold and exhaustion. The end of the valley was in sight and he knew they had covered more than a hundred miles in one ride. When he had been young, he might have laughed at such a challenge, but at forty, his knees and ankles had begun to hurt with every stride of his mount. His men too were weary, though they had the grim endurance of desert Arabs. They lifted their heads as the order came to close the gap once more. Surely he could bring the Mongols to battle this last time!

There was no sudden surge of speed to alert the enemy ahead. Instead, Khalifa urged his panting mount on slowly, closing the gap to just four hundred paces before the Mongols reacted. Khalifa raised his hand then, roaring through the dust in his throat for a charge.

His men dug in their heels and the exhausted horses responded, hitting a ragged gallop. Khalifa heard a horse scream and go down, spilling a man to the ground. He could not see what had happened as he closed to two hundred paces and drew a long, black arrow from the quiver on his back.

The Mongols had seen the threat and answered it with a volley of shafts loosed behind them as they galloped. Even then, the accuracy was terrifying and Khalifa saw men and horses plucked away to be trampled on every side. He snarled in frustration as his arrow feathers touched his cheek. His mount was foundering and still they managed to widen the gap. He let go, crying out in triumph as his shaft took an enemy high in the back, sending him crashing down. Dozens more were struck, though armour saved some. Those who fell went under the Arab hooves as they writhed in the dust, struck many times until their bones were shattered pulp.

Khalifa shouted raucously to his men, but they were finished. He could see from the way they swayed in the saddle that they had reached the end of their strength. Many of the horses had gone lame in the night. They drifted to the rear as their riders flailed uselessly with whips and sword scabbards.

He considered ordering a halt, but the effort was too much. Always, he thought he could hang on a little longer, just until the Mongols killed their horses and began to die themselves. His eyes were sore and red from the gritty dust he had ridden through all night and he could only watch as the enemy drew ahead once more, to half a mile and further. There they stayed as the sun rose higher and neither side could widen or close the gap. Khalifa put his bow back in the leather sleeve behind his right leg and patted his horse’s neck.

‘Just a little further, great heart,’ he murmured to the plunging animal. He knew that many of the horses would be ruined after such a ride. They had been pushed beyond anything they had known before and the wind of many would be permanently broken. He heard another thump and cry as a horse fell somewhere behind him, staggering into those around it and collapsing. Others would follow, he knew, but still the rear ranks of the Mongols beckoned him on and he narrowed his eyes against the choking dust.

As the tumans came out of the shadowy valley onto a plain, their spirits lifted. They could see the morning smoke of villages in the distance and they followed a road of packed earth into the east. Somewhere ahead lay cities of the shah and potential reinforcements for those who still followed. Jebe and Jochi had no idea how many men the shah could bring to the field. His cities could have been stripped for the war, or left well-manned and bristling for just such a raid into their territory.

The road was wide, perhaps because of the huge army that had trampled the earth in passing, just a few days before. The Mongol column narrowed to use the hard ground, riding in ranks of fifty across as they came out of the mountains in a whirl of dust. The sun passed noon and the heat brought horses and men crashing down on both sides, vanishing behind in a welter of hooves. The Mongols sweated and there was no water or salt to keep up their strength. Jebe and Jochi began to glance back more and more often in desperation.

The Arab horses were better than anything they had faced before in war, certainly better than Chin or Russian mounts. Yet as the heat sapped their strength, the pursuers began to fall further behind until Jebe ordered a slower pace. He did not want to lose them or allow them time to halt and regroup. He thought perhaps that they had led the shah’s riders for more than a hundred and fifty miles, approaching the limits of even the toughest Mongol scouts. The ponies were lathered in strings of soapy spit, their skin dark with sweat and fresh sores where the saddle had rubbed away patches of old callus.

Long into the sweltering afternoon, they passed a road fort with open-mouthed soldiers on the walls, shouting challenges to them as they passed. The Mongols did not respond. Each man was lost in his own world, resisting the weakness of flesh.

Jochi spent the hot hours in pain as a raw spot appeared on his thigh, rubbed bloody in the ride. It went numb as the evening came once again, which was a blessed relief. His scars had eased, but his left arm felt weak and the ache there had become a hot iron in his flesh as he gripped the reins. There was no talking in the Mongol ranks by then. Their mouths were closed as they had been taught, conserving moisture in their bodies as they approached the end of endurance. Jochi looked to Jebe occasionally, waiting for the other man to judge the best time to break off the ride. Jebe rode stiffly, his eyes hardly leaving the horizon ahead. To look at him, Jochi thought the young general might well ride to the horizon.

‘It is time, Jebe,’ Jochi called to him at last.

The general stirred sluggishly from his daze, mumbling something incoherent and spitting feebly, so that the wad of phlegm struck his own chest.

‘My Chin warriors are drifting further back,’ Jochi went on. ‘We could lose them. Those who follow are letting the gap widen.’

Jebe turned in the saddle, wincing as his muscles protested. The Arabs were almost a full mile behind. The lead animals were stumbling and lame and Jebe nodded, a tired smile crossing his face as he came fully alert.

‘At this pace, a mile is only four hundred heartbeats,’ he said.

Jochi nodded. They had spent part of the dawn judging speed with markers as they passed them and then took note of the Arab ranks drawing abreast that point. Both Jochi and Jebe found the calculations easy and had amused each other estimating distance and speed to pass the time.

‘Increase the pace then,’ Jochi replied. He forced his mount to a canter as he spoke and the tumans matched them doggedly. The enemy dwindled with painful slowness as the generals called out the mark. When the first Arab riders passed a pinkish stone six hundred heartbeats after the last Mongol, the generals looked at each other and nodded grimly. They had come as far as any scout had ever ridden and further. All the men were weary and sore, but it was time. Jochi and Jebe passed orders down the line so that the warriors were ready. Though they had pushed themselves to the limit, Jochi and Jebe both saw something in the red-rimmed eyes of those around them that made them proud.