At first, the shields protected his men, but as they reached the last hundred paces, the air was thick with whistling shafts and men died in scores. Khalifa’s shield was of wood, covered in the dried hide of a hippopotamus – the lightest and best of all the shah’s equipment. It held, though the muscles of his arm were bruised and battered until he could barely hold it. Without warning, he felt his horse shudder and begin to die.

Khalifa would have leapt clear, but his feet snagged in the stirrups and, for a breathless moment of panic, his right leg was trapped under the dying horse. Another mount crashed into his as it fell and he jerked free, thanking Allah for his deliverance. He rose on sandy ground, spitting blood and wild with rage.

The entire front rank had been brought down by the archers, fouling those behind. Many of his men were yelling, tugging at shafts through their legs and arms while others lay sprawled and unmoving. Khalifa roared fresh orders and the men behind dismounted to lead their mounts through the broken dead. The gap closed further and Khalifa held his sword high, pointing it at the enemy above. One hundred paces and he was lost in his desire to kill. If anything, he was faster on foot, though every step on loose ground sapped his strength. He scrambled up with his sword ready for the first blow. The shah was watching and Khalifa could almost feel the old man’s eyes on his back.

The Mongols poured over the ridge, straight down the steep slope. Their ponies slid, with front legs straight and stiff while the back legs bunched to keep them upright. The desert warriors strained to take the first impact, but to Khalifa’s shock, another wave of arrows punched them from their feet before the two forces met. He could not understand how the Mongols could draw and loose while guiding their mounts down such a slope, but the volley devastated his men. Hundreds died on foot or leading their mounts and this time the shafts were followed by the Mongol front line crashing down on them. Khalifa heard their yelling swell until it seemed to echo back from the hills all around.

The Mongol horsemen came like a breaking wave, smashing anything in their path by sheer weight. Khalifa was standing behind the bodies of two horses and could only watch in astonishment as the charge roared past him, a wedge tipped with lances that struck deep and deeper into the climbing lines below.

He was left alive, but still they came. Khalifa could not climb further. The way was blocked by thousands of Mongol horsemen, guiding their mounts with just their knees while they loosed arrows at anything that moved. A long shaft ripped through his side, parting the steel links of his armour as if they had been made of paper. He fell, shouting incoherently, and it was then that he glimpsed another force cutting across the face of the slope.

Jochi’s men hit the flank of the Arab riders below Jebe’s charge. Their arrows tore a hole in the ranks and they followed it with lances and swords, cutting men down while they were held in the press. Khalifa stood to see them, fear and bile rising in his throat. Arrows still whirred by his bare head, but he did not flinch. He saw the two forces meet in the centre and the combined mass drove his men further down so that they almost reached the valley floor. Bodies covered the ground behind them and riderless horses ran wild, knocking fresh warriors from their saddles in their panic.

The Mongol charge from the ridge had passed him by and Khalifa saw one horse with its reins trapped under a dead man. He ran to it, ignoring the pain in his side as he mounted, throwing his shield aside with a curse when the arrow shafts snagged. The air was thick with dust and the cries of dying brothers, but he had a horse and a sword and had never asked for more. Perhaps thirty thousand desert men still lived, struggling below to hold back the twin charge. Khalifa could see the Mongols had gambled their full force in the attack and he shouted as he rode wildly down the hill towards the ranks. They could be held. They could be broken, he was sure of it.

As he reached his men, he bellowed commands to the closest officers. A solid square began to form, ringed with shields. The Mongols threw themselves at the edges and began to die as they met the swords of his tribe. Khalifa felt the battle like a live thing and knew he could still turn the losses to triumph. He had his men retreat in order back to the flat ground, harried all the way by the Mongol warriors. He drew them away from the slope they had used to such effect, and when the earth was hard under his mount, Khalifa ordered a charge into them, urging his men on with words of the prophet.

‘They shall be slain or crucified or have their hands and feet cut off on alternate sides, or be banished from the land. They shall be held up to shame in this world and sternly punished in the hereafter!’

His men were true Arabs of the blood. They heard and became fierce once more, taking the fight to the enemy. At the same time, the shah moved at last, sending fresh soldiers racing in squares as the Mongols came within range. The lines met and a roar went up as the Mongols were knocked back, defending desperately as attacks came from more than one direction. Khalifa saw the shah’s ranks move wide to surround them, marching steadily in.

The Mongol warriors faltered, overwhelmed as Khalifa barged his horse through to the front rank. A young warrior came at him and Khalifa braced and took the man’s head as he swept past. The shah’s riders advanced, their swords red. Discipline held them and Khalifa was proud. Once more, he sensed uncertainty in the attacking horsemen and suddenly they broke and ran, leaving the foot regiments in their wake as they galloped clear.

Khalifa ordered his lancers forward and was pleased with their formation as they took many of the fleeing men in the backs, hammering them from the saddles.

‘For the prophet, brothers!’ he roared. ‘Run these dogs down!’

The Mongol warriors were streaming across the plain on their ponies, riding flat out. Khalifa raised his hand and dropped it and the lines of Arabs dug in their heels to give chase. They would pass along the flank of the shah’s army and Khalifa hoped the fierce old man would see and give thanks. As he rode, he glanced back to the slope leading up to the ridge. It was black with the dead and he felt new strength surge in him. These men had dared to enter his land and they would find only fire and the sword.

After the initial race east along the valley, both the tumans and their pursuers settled into a slow gallop that ate the miles. Before the sun set, Khalifa’s men tried to close the gap three times and were driven back with arrows fired by men turning in the saddle. Unlike the Mongols, the Arab horsemen were not accurate loosing arrows at full speed. Though their mounts were faster over short distances, they were forced to settle in for a long chase. By the time the sun touched the west behind them, they were more than a dozen miles from the shah’s army. The Mongol warriors rode in grim concentration, knowing that to fall behind was to die.

Jochi and Jebe had come together about halfway through the ranks of their men. They did not know how many of their number had been lost on the slopes under the ridge. The Arabs had fought well at the end, but both generals were pleased with what they had achieved. Genghis would be told of both strength and weaknesses in the enemy and what they had learned would be vital to the khan in the days to come. Still, they had to survive the dogged pursuit. Both men knew it was easier to chase than to be hunted. As eagles and wolves had eyes to the front of their heads, so did man. Riding after an enemy kept the spirits strong, just as hearing the enemy always at their backs sapped the confidence of the tumans. Yet they did not falter.