'What was that?' he demanded.

One of his bondsmen answered immediately. 'Gunpowder, my lord. They have fire-pots.'

'In the field?' Khasar said. He cursed loudly. He had seen the weapons used on city walls and he knew their effect. Iron pots filled with black powder could rip shards of hot metal through the packed ranks of his men. They had to be thrown far enough for the defenders not to be torn apart themselves. He could not imagine how the Chin were using them without killing their own people.

Before he could gather his stunned thoughts, another great crack sounded. At a distance, the sound was muted, but he saw men and horses blown back from the explosion, landing brokenly on the grass. The smell came to him then, acrid and bitter. Some of his men coughed in the breeze. The Chin cheered with more energy and Khasar's face became savage.

Every instinct urged him to gallop towards the enemy before they could use the small advantage they had won. Ogedai's advance had lost its momentum and only the edge of the two armies were in contact, like struggling insects over the distance. Khasar forced himself under control. This was no raid against tribesmen. The Chin had numbers and nerve enough to lose half their men just to gut the khan of the Mongols. The sky father knew the Chin emperor had the desire. Khasar felt his men staring at him, waiting for the word. He clenched his jaw, grinding his teeth.

'Hold. Wait,' he ordered, watching the battle. His two thousand could mean the difference between victory and defeat or just be lost in the mass. The choice, the decision, was his. Ogedai had never heard thunder like it. He had been riding well back in the ranks as the armies came together. He had roared as the arrows flew, thousands at a time, again and again before his warriors drew swords and hit. The men around him had surged forward, each man keen to show courage and win the khan's approval. It was a rare opportunity for them to be within sight of the man who ruled the nation. No one wanted to waste it and they prepared to fight like maniacs, showing no pain or weakness.

As they went forward, a thumping blast threw men backwards and left Ogedai's ears ringing. Dirt spattered him as he tried groggily to understand what had happened. He saw a man unhorsed, standing numbly with blood running down his face. A small group lay dead, while many more twitched and pulled at metal stuck in their flesh. The explosion had deafened and stunned those closest. As the ranks surged forward, Ogedai saw one unhorsed man stumble into the path of a rider and go down under the hooves.

Ogedai shook his head to clear the sound of rushing air, the emptiness. His heart pounded in his ears and a wide band of pressure had grown in his head. He thought of a man he had once seen tortured, a flashing image of leather thongs bound around the head and tightened with a stick. It was a simple device, but it produced appalling agony as the skull shifted and eventually broke. Ogedai's head felt like that, as if the band were slowly tightening.

Another booming explosion seemed to raise the ground beneath them. Horses squealed and reared, their eyes wild as the warriors fought savagely to control them. Ogedai could see black specks hurled high into the air from the Chin forces. He did not know what they were, or how to counter them. With a sudden shock that cut through even his drunken state, he realised he could die on that stony plain. It was not a matter of courage or even endurance, but mere luck. He shook his head again to clear it and his eyes were bright. His body was weak, his heart feeble, but above all things, he had luck. Another crack whipped across the field, followed by two more. Ogedai's men were wavering, shocked into immobility. To his right, Tolui's tuman had gone further, but they too were stunned by massive explosions that killed men on both sides.

Ogedai drew his father's sword in one swift gesture, bellowing defiance as he held it up. His bondsmen saw his recklessness and it lit their blood. They came with him as he kicked his mount forward, already grinning at the maniac khan, charging the enemy on his own. They were all young men. They rode with the most beloved son of Genghis, marked by the sky father, khan of the nation. Their lives were not worth as much as his and they threw them away as carelessly as they would a broken rein.

The explosions came faster as more of the black balls were flung sputtering into the air to land around Mongol feet. As he thundered forward, Ogedai saw one unhorsed warrior pick one up. The khan shouted, but the man was blown into a bloody mass. The air was suddenly full of whining flies. Horses and men screamed as needles of iron tore into them all around.

Ogedai's bondsmen plunged into the fray, protecting their khan at the centre. Lowered pikes stopped the horses, but more and more of his men had been unhorsed and they killed the pike-wielders with knives and swords, clearing a path as horses shoved and sweated at their backs. Ogedai saw another black ball drop almost at his feet and one of his men threw himself on top of it. The whump of sound was muted, though a small red crater appeared in the man's back and a piece of bone jumped out, almost to the height of a man. Those around Ogedai flinched, but they stood immediately straight, ashamed that the khan might have seen their fear.

Ogedai realised he had seen some sort of answer to the weapons. He raised his voice to carry across the lines.

'Fall on them as they land, for your khan,' he shouted.

The order was repeated down the lines as the next wave of missiles were thrown high. There were six of the flying iron balls, each fizzing with a short fuse. Ogedai watched in pride as warriors struggled to reach them, smothering the threat so that their friends might live. He turned back to the enemy and saw fear in the Chin faces. There was only vengeful fury in his own.

'Bows!' he roared. 'Clear a path and bring up lances. Lances here!'

There were tears in his eyes, but not for those who had given their lives. There was joy in every waking, breathing moment. The air was cold and bitter in his throat, filled with the strange smell of burning powder. He breathed it deeply, and for a time the band across his face and head seemed to ease as his men ripped a gash in the Chin ranks. Khasar beat his fist against his armour in unconscious approval of Ogedai's manoeuvres. The two Mongol tumans had been rocked back by the explosions, moving instinctively away from the source of such sound and snaps of light. Khasar had seen the khan's own bondsmen overcome their fear and hack open the Chin lines. The thumps of explosions were suddenly muffled and he no longer saw the spray of stones and dirt each time one of them went up. It was as if they dropped into the Mongol army and were swallowed. He grinned at the thought.

'I think the khan is eating those iron balls,' he said to his men. 'Look, he is still hungry. He wants more to fill his stomach.' He hid his own fear at such a reckless charge from Ogedai. If he died that day, Chagatai would rule the nation and everything they had struggled for would have been in vain.

His experienced eye ran over the battlefield as he trotted his horse south, keeping them in range. In that at least, the Chin emperor had not wavered. His men moved as quickly as they could, struggling over the dead as they marched. Such an army could not easily be stopped by half its number. It was a tactical problem and Khasar struggled with it. If he ordered thinner lines spread like a net, the Chin could break through with a spear thrust. If he kept the depth of men, they could be passed on the flanks as the emperor forced each dogged step towards the border. It must be an agony for him, Khasar thought, to be so close and yet have an enemy boiling around him.

His own minghaans killed almost at their leisure in the enemy rear, leaving a trail of bodies on the rough grass. The Chin would not turn, so intent were they on reaching the border. As he trotted south behind them, Khasar came across one soldier draped on the limbs of a low thorn tree. He glanced at the man and saw his face twitch and the eyes open in sightless agony. Khasar reached out with his sword and flicked the tip across the throat. It was not mercy. He had not killed that day and he longed to be part of the battle.