It was a spring morning when Tsubodai gathered his most senior officers on a meadow by a lake filled with wildfowl. His scouts had spent the morning trapping thousands of birds in nets, or taking them in flight for sport. The women in the camps were plucking the birds to be roasted that evening, creating great drifts of feathers that tumbled over the grass like spilt oil.

Batu watched with carefully hidden interest as Tsubodai brought forward one of his strongest warriors. The man's face could not be seen under the helmet of polished iron. Everything he wore had been captured further to the west. Even the horse was a monster, black as night and half as high again at the shoulder as any Mongol pony. Like its rider, it was sheathed in iron, from plates around its eyes to a skirt of hardened leather and metal to protect its hindquarters from arrows.

Some of the men looked on it with greed in their eyes, but Batu scorned such a beast. As large as it was, with such a burden of armour he was certain it would be slow, at least in the thrust and parry of battle.

'This is what we will face as we move west,' Tsubodai said. 'Men like this in cages of iron are the most feared force on a battlefield. According to the Christian monks in Karakorum, they are unstoppable in the charge, a weight of metal and leather that can crush anything we have.'

The senior men shifted uncomfortably, unsure whether to believe such a wild claim. They watched in fascination as Tsubodai brought his pony close to the larger animal. He looked small next to the man and horse, but he used his reins lightly to take his pony around in a tight circle.

'Raise your hand when you can see me, Tangut,' he said.

It was not long before they understood. The line of sight Tsubodai had revealed was just a small strip to the front.

'Even with the visor raised, he can see nothing at the side or behind, and that iron will be hard to turn quickly.' Tsubodai reached out and clanged his fist against the warrior's breastplate. It rang like a bell.

'His chest is well protected. Under this is a layer of iron links, like metal cloth. It serves a similar purpose to our silk tunics, but is made to withstand axes and knives more than arrows.'

Tsubodai gestured to a boy holding a long lance and the boy ran to the armoured warrior and handed it up to him, tapping his leg for attention.

'This is how they are used,' Tsubodai said. 'Like our own heavy horse, they ride head-on against an enemy. In a charge, they have no flaw or hole in their armour.'

He nodded to Tangut and they all watched as the warrior trotted away, his ungainly metal carapace jingling with every step.

At two hundred paces, the man turned his heavy mount, which reared and flattened its ears. He dug in his heels and the animal lunged forward, the thick legs thumping the ground. Batu saw how dropping the horse's head brought the armour of chest and skull together, forming an impenetrable shell. The lance lowered, the point cutting the air in circles as it centred on Tsubodai's chest.

Batu found he was holding his breath and he let it go, annoyed with himself for falling under Tsubodai's spell. He watched coldly as the warrior hit full gallop, his lance a deadly weapon. The hooves thundered and Batu had a sudden vision of a line of such men sweeping across a battlefield. He swallowed at the thought.

Tsubodai moved quickly, darting to one side with his pony. They saw the armoured warrior try to correct, but he could not turn at full speed and swept past.

Tsubodai raised and drew his bow in a fluid motion, aiming casually. The front of the horse was as well armoured as its rider. There was even a crest of armour running along the line of the mane, but below that the great neck was open and bare.

Tsubodai's arrow punched into the flesh and the horse screamed, spattering bright blood from its nostrils.

'From the sides, to a good bowman, they are unprotected,' Tsubodai shouted over the noise. He spoke without pride: any one of the men watching could have made the shot. They smiled at the thought of such powerful enemies brought down by speed and arrows.

They could all hear the tortured snorting of the horse as it wrenched its head back and forth in pain. Slowly, it sank to its knees and the warrior stepped clear. He dropped his lance and drew a long sword, advancing on Tsubodai.

'To defeat such armoured men, we must first kill the horses,' Tsubodai went on. 'Their armour is designed to deflect arrows shot from the front. Everything is made for the charge, but on foot, they are like turtles, slow and ponderous.'

To make his point, he selected a thick arrow with a long steel tip. It was a wicked-looking thing, smooth and polished, with no barbs to slow its speed.

The approaching warrior saw the action and hesitated. He did not know how far Tsubodai was willing to go to make his point, but the general would be equally ruthless with a man whose nerve failed. The moment of indecision passed and the warrior clumped forward, straining to pump his heavy legs and arms quickly so that he could bring his sword to bear.

From the saddle, Tsubodai guided his mount with his knees, sending the pony dancing back out of sword range. He drew again, feeling the immense power of the bow as he pulled the three-foot shaft right back to his ear. With the warrior just a few paces away, Tsubodai loosed and watched closely as the shaft passed straight through the side plates.

The warrior went down hard with a crash of metal. The arrow was lodged in his armour, the feathers showing clearly as he toppled. Tsubodai grinned.

'They have just one strength – in a line, facing forward. If we allow them to use that strength, they will sweep us away like wheat to a scythe. If we scatter and ambush them, stage false retreats and flank them, they will be as children to us.'

Batu watched as Tsubodai's serving men carried the dying warrior away between them, sweating and struggling under the massive burden. At a distance, they stripped the armour, revealing a mailed body spitted through by the shaft. They had to break the arrow to get the plate free and bring it back to Tsubodai.

'According to those boastful Christians who wished to frighten us, these knights have had no equal on the battlefield for a hundred years.' He held up the metal plate and everyone there could see the sunlight gleam through the neat hole. 'We cannot leave a major force or city behind us or on our flanks, but if this is the best they have, we will surprise them, I think.'

They raised their own bows and swords then, cheering Tsubodai's name. Batu joined them, careful not to be the only one who remained outside the group. He saw Tsubodai's gaze flicker over him. A look of satisfaction crossed the general's face at seeing Batu shout with the others. Batu smiled at the thought of holding Tsubodai's head up in just such a way. It was only a fantasy. The army was strong, but he knew they needed Tsubodai to lead them west against the great armies of horsemen and further to these men of iron. For Batu, men like Tsubodai were old and approaching the end of their time. His chance would come naturally; he did not need to force its progress. Chagatai had built a summer palace on the banks of the Amu Darya river, the western edge of his empire that led as far south as Kabul. For the site, he had chosen a high ridge over the river where there was always a cool breeze, even in the hottest months. The sun of his khanate had baked him lean and dark, as if all the moisture had been boiled out, leaving him as hard as ancient birch. He ruled the cities of Bukhara, Samarkand and Kabul, with all their wealth. The citizens there had learned to deal with the summer heat, sipping cool drinks and sleeping through the afternoon before rising again. Chagatai had chosen almost a hundred new wives from those cities alone and many of them had already given birth to sons and daughters. He had taken Ogedai's order to breed a new army literally and he enjoyed the sound of the squalling children in the infant rooms of his seraglio. He had even learned the new word for his collection of beautiful women, as there was nothing like it in his own tongue.