They did not move and Xuan reached down to brush a speck of dust from his gloves. He showed no discomfort as he stared over the heads of the Sung army. He could hear the Mongols ripping at his own men, but he did not move or acknowledge it. There was a chance that his cousin Lizong would allow his army to be destroyed while they all waited. Xuan seethed at the thought, but there was nothing he could do. He had come as a supplicant to the Sung lands. If the emperor chose to remove his strength in such a way, Xuan knew he could not react. It was a bold stroke and he could almost applaud it. Let the damaged Chin emperor enter, but let him see his army withered to just a few men first. Let him come on his knees, begging for favour.

All Xuan's choices, all his plans and stratagems, had been reduced to one course of action. He had ridden up to the lines. If they opened to let him in, he could pass to safety with whoever remained alive in his army. Xuan tried not to think what might happen if his poisonous Sung cousins had decided to remove him from the balance. It was not beyond them to have manoeuvred him to exactly this position, waiting, waiting, waiting. He could sit his horse in front of them until the Mongols had finished slaughtering his army and came for him. There was a chance Lizong would not lift a hand to save him even then.

Xuan's face was utterly without emotion as he studied the Sung soldiers. Whatever happened was his fate and not to be denied. Some hidden spark of him was white with fury, but nothing showed. As casually as he could, he turned to one of his generals and asked about the cannon the Sung had used.

The general was sweating visibly, but he replied as if they were at a military inspection.

'It is a field-piece, imperial majesty, similar to the ones we have used on city walls. Bronze is poured into a mould and then filed and polished. Black powder burns with great fierceness, sending a ball leaping out to cause terror in the enemy.'

Xuan nodded as if he was fascinated. By the spirits of his ancestors, how long must he wait?

'Such a large cannon would be very heavy,' he said stiffly. 'It must be difficult to move over rough terrain.'

The general nodded, pleased that his master had engaged him in conversation, though he knew the stakes as well as anyone.

'It sits on a wooden cart, imperial majesty. It is wheeled, but yes, it takes many men and oxen to drag it into position. More are needed to carry the stone balls, the powder bags, the swabs and fuses. Perhaps you will have the chance to inspect one more closely when we enter Sung territory.'

Xuan looked at the general in reproof for his lack of subtlety.

'Perhaps, general. Tell me now about the Sung regiments. I do not know all these banners.'

The man began to recite the names and histories, an expert in his field, as Xuan knew very well. He cocked his head to listen to the droning voice, but all the time, Xuan watched the Sung lines. The Son of Heaven glanced up as an officer rode a magnificent stallion through to the borderline. He tried not to show how his heart leapt.

It was hard to allow his general to finish the litany of names, but Xuan forced himself to listen, making the Sung officer wait for both of them. His precious army was being butchered as he nodded at tedious detail, but Xuan's face was calm and interested.

At last, his general had the sense to subside and Xuan thanked him, appearing to notice the Sung officer for the first time. The man dismounted as soon as their eyes met. He came forward and prostrated himself on the dusty ground before touching his forehead to the general's stirrup. He did not look at Xuan as he spoke.

'I bear a message for the Son of Heaven.'

'Speak your message to me, soldier. I will tell him,' the general replied.

The man prostrated himself again, then rose. 'His imperial majesty bids you welcome in his lands, Son of Heaven. May you live ten thousand years.'

Xuan would not lower himself to reply to a mere soldier. The message should have been delivered by someone of noble rank and he wondered what to make of the subtle insult. He barely listened as his general completed the formalities. Xuan did not glance behind him as he walked his mount forward. Sweat trickled down his back and from his armpits under his armour. He knew his undertunic would be sopping wet.

The Sung lines stood apart as he moved, a rippling motion that spread along them for half a mile. In this way the last Chin army could walk between the ranks and the border was still held against their mutual enemy. Xuan and his generals crossed the invisible line, showing no emotion to those who watched them. The Chin ranks began to follow them, like a blister collapsing into skin. Ogedai watched in furious disbelief. He saw pavilions rise amidst the Sung ranks, great squares of peach-coloured silk. Banners floated on the wind, marking out regiments of bowmen, pikemen, lancers. It was the sight of the fresh cavalry that broke through his battle madness. Regiments of horsemen stared out onto the broken plain with its trail of dead. Would the Sung be able to resist a sudden charge, as soon as the Chin emperor was safe? Only the setting sun would stay their hand and perhaps not even then. The Mongol ponies had ridden for days. They were as weary as their riders and, for once, the khan himself was in the field, vastly outnumbered and with every advantage taken away. Ogedai shook his head. He had seen the puff of smoke that revealed the presence of heavy guns. It was a thought for another day, but he did not see how he could ever bring such weapons to a battlefield. They were too slow, too heavy for an army whose chief strength had always been in its speed and manoeuvres. In the distance, he saw a small group of horses move through the Sung lines. Perhaps ten thousand still marched to follow them, but the Chin emperor had passed through the net.

Ogedai felt a wave of weariness replace the thrilling energy of the fighting. He could hardly believe he had walked without fear. He had faced his enemies and survived unmarked. For just an instant, a heartbeat, pride swelled in him.

Even so, he had failed. The band across his head returned, tightening. He imagined mockery in every concerned face. He could almost hear the whispered voices among his warriors. Genghis would not have failed. His father would somehow have plucked victory from disaster.

Ogedai gave fresh orders and the three tumans pulled back from the retreating Chin ranks. The men had been expecting the command and the minghaans moved quickly and easily into squares of horse, facing the Sung border.

The sudden silence was like a pressure and Ogedai rode slowly along the lines of his own men, his face flushed and sweating. If the Sung generals wanted him badly enough, they would not even wait for the rest of the Chin to come across. Half the Sung army could launch an attack at that moment. Ogedai swallowed, working his tongue around a mouth so dry he thought he would choke. He gestured to a messenger and the man brought him a skin of red wine. It moistened his lips and he gulped at it, sucking desperately on the leather teat. The pain in his head was growing all the time and he realised his vision was blurred. He thought at first it was just sweat in his eyes, but it remained no matter how roughly he rubbed at them.

As the Mongol tumans came to order, hundreds were still panting or binding gashes. Ogedai saw Tolui trotting a mare across the broken ground towards him. The two brothers met with a quick glance of resignation and Tolui turned his mount to watch the Chin emperor escape them once again.

'He has a lot of luck, that man,' Tolui said softly. 'But we have his land and his cities. We have taken away his armies except for that rabble of survivors.'

'Enough,' Ogedai snapped, rubbing his temples. 'You do not need to add honey. I must bring an army into Sung lands now. They have given sanctuary to my enemies and they know I must respond.' He winced and sucked again on the wine-skin. 'There will be other days to avenge the dead. Form the men up to go back to the north, with haste, but not too visible, do you understand?'