Jochi had sent orders to the minghaan officers of his Chin recruits at the back and it was one of those men who rode up through the ranks to speak to him.

The Chin soldier was covered in dust as thick as paint, so that cracks appeared around his eyes and mouth. Even then, Jochi could see his anger.

‘General, I must have misunderstood the order you sent,’ he said, his voice a dry croak. ‘If we turn to face this enemy, my men will be in the front rank. Surely you cannot mean to have us fall back?’

Jochi glanced at Jebe, but the Mongol general had fixed his gaze on the horizon.

‘Your men are exhausted, Sen Tu,’ Jochi said.

The Chin officer could not deny it, but he shook his head.

‘We have come this far. My men will be shamed if they are taken from the line of battle at the end.’

Jochi saw fierce pride in his officer and realised he should not have given the order. Many of the Chin would die, but they too were his men to command and he should not have tried to spare them.

‘Very well. You have the first rank when I call the halt. I will send those with lances to you. Show me you are worthy of this honour.’

The Chin officer bowed in his saddle, before returning to the rear. Jochi did not look again at Jebe, though the latter nodded in appreciation.

It took time for the orders to spread through the Mongol riders. For tired men, it seemed to act like a gulp of airag, so that warriors sat straighter in their saddles and readied their bows, lances and swords. While they still rode, Jebe sent his lancemen to support the rear and waited until they were in position.

‘We have come a long way, Jochi,’ Jebe said.

The khan’s son nodded. He felt as if he had known Jebe all his life after the night ride.

‘Are you ready, old man?’ Jochi said, grinning despite his tiredness.

‘I feel like one, but I am ready,’ Jebe replied.

Both men raised their left hands high into the air and circled their fists. The Mongol tumans ground to a halt and the gasping horses were turned to face the enemy riding towards them.

Jebe drew his sword and pointed it at the dusty Arab riders.

‘Those are tired men,’ he roared. ‘Show them we are stronger’

His mount snorted as if in anger and broke into a gallop, its sides heaving like bellows as they charged the pursuing enemy.

Khalifa rode in a daze, drifting in and out of alertness. At times, he thought of the vineyard near Bukhara, where he had first seen his wife tending the crop. Surely he was there and this ride was just a fever dream of dust and pain.

His men began to shout with dry throats all around him and Khalifa raised his head slowly, blinking. He saw the Mongols had stopped and for a moment he took a searing breath in triumph. He saw the rear ranks raise lances and suddenly the gap between the armies was closing. Khalifa hardly had strength to speak. When he tried to shout, his voice was a feeble whisper. When had he emptied his water flask? That morning? He could not remember. He saw the approaching line and somehow Chin faces were grinning at him. Even then he could barely raise his shield.

The approaching lancers carried small shields in their left hands, some part of him noted. Archers needed both hands for the bows and were vulnerable just as they began to draw. Khalifa nodded to himself at the thought. The shah would value such information.

The two armies came together with a numbing crash. The heavy birch lances broke shields and pierced men right through. On the narrow road, the column ripped into the Arab riders, deeper and still deeper, tearing them apart.

Arrows screamed past his ears and Khalifa felt something burn his stomach. As he looked down, he saw an arrow there and he plucked at it. His horse had stopped moving at last, falling to its knees as its heart burst in its chest. Khalifa fell with it, the cursed stirrups entangling his right leg, so that his knee tore and his body twisted as he fell. He gasped as the arrow drove further through him. Above his head, he could see Mongols riding like kings.

Khalifa could hear nothing but wind rushing in his ears. The Mongols had ridden them down and he feared for the armies of the shah. He must be told, Khalifa thought to himself, but then he was gone.

‘Kill them all!’ Jochi shouted above the roaring hooves and men.

The Arabs tried to rally, but many could barely lift their swords more than once and they fell like wheat. The generals smashed through them with their column, seeming to take new strength from every man they killed.

It took hours to turn the dusty road red. As it grew dark, the slaughter continued until they could not see to strike and those who tried to run were brought down by shafts, or chased like lost goats. Jebe sent scouts to look for water and at last they made camp on the banks of a small lake just three miles further down the road. The warriors had to be watchful then, as their mounts would have drunk to bursting. More than one had to strike his pony hard on the nose to stop it taking too much water. Only when the animals had drunk did the men throw themselves into the lake, turning the dark waters pink with blood and dust as they gasped and drank and vomited it back up, cheering the generals who had brought them such a victory. Jochi took the time to commend Sen Tu for the way he had led the Chin recruits. They had hacked through the enemy with unmatched ferocity and they sat at fires with tribesmen of both tumans, proud of the part they had played.

Jochi and Jebe sent aching men back along the road to quarter dead horses and bring them to the fires. The men needed meat as much as water if they were to make it back to Genghis. Both generals knew they had done something extraordinary, but they fell into the routines of the camp with just a shared glance of triumph. They had deprived the shah of his cavalry wings and given Genghis a fighting chance.

The gates of the city of Otrar were barred against Genghis. He sat his pony on a hill overlooking the city, watching dark smoke lift sluggishly over the burning suburbs. He had spent three days scouting the land, but even for those who had taken dozens of Chin cities, there was no obvious flaw in the design. The walls had been built in layers of light grey limestone on a granite base, each slab weighing many tons. In the walls of the inner city, two iron gates led out to a sprawling maze of abandoned markets and streets. It had been strange to ride through those echoing passageways in sight of the great walls. The governor had known they were coming for months and, apart from a few stray dogs and broken pots, everything of value had been taken. Genghis’ scouts had found a number of subtle traps set for them as they searched. One boy of only thirteen had kicked open a door and fallen back with a crossbow bolt in his chest. After two more deaths, Genghis had given Temuge the task of firing the outer city and Otrar still choked on black smoke. In the cinders and rubble below the hill, Tsubodai’s Young Wolves used pikes to pull down the walls and give the khan a clear route to the inner city.

There was no shortage of information. In exchange for gold, Arab merchants even gave the location of wells within the walls. Genghis had ridden round the entire city with his engineers, noting the thickness of the stone.

The clearest weakness was the hill on the city’s northern side, overlooking the walls. His scouts had found abandoned pleasure gardens there, rich with flowers and an ornamental lake and wooden pavilion. Two days before, Genghis had sent warriors to clear the crest, leaving the rest covered in ancient pine trees. If he sited his wall-breaking weapons where the pavilion had stood, they would have the height to send stones right down the throat of the governor.