Zhou lords had been raising horses to pull chariots for hundreds of years, as shown by spectacular burials that include up to 14 horse skeletons along with a chariot. But fighting from horseback is a completely different skill, developed later in world history. Warring States armies learned about cavalry from their northern neighbors, the Hu. These nomadic clans headed by mounted archers lived in the upper Ordos area (under the bump in the Yellow River). They first appear in Zhou records in 457 BC. The Hu sold or granted to the Zhou states horses, other animals, furs, and wine and millet made by the farming people they dominated. In return, Zhou states sent gold and silver items, belts with precious shells, pearls, sometimes a matched team of four horses, and silk. And women: as with relations between Zhou states, the exchange of wives signaled and solidified political alliance, for children would then be descended from both sides. Northern excavations have found gold plaques with characters recording their weight, and hoards of coins. The coins signal an extensive monetized trade, as the mounted aristocrats of both the Zhou and the Hu added control of trade and production to their rule by violence.
Zhou states were willing to pay for horses because they were the new war machines of the day. In 307 BC, the king of Zhao was expanding northwards. He wanted to train his armies in mounted archery, which meant wearing trousers instead of the traditional gown. When the aristocrats of Zhao objected to adopting foreign clothing, the king reminded first them of the long practice of deploying foreigners against other states, and second that sage-kings and former rulers had changed their ways to suit the times. Third, the Zhao king argued that it was not right to condemn other cultures – an important perspective, since the various Zhou states themselves were increasingly diverging in culture. Fourth, he argued that tools must fit the need; just as one uses boats on rivers, so to fight in hilly lands one needs horses. To implement his policy, he relied on northern locals in his new territory: men who knew neighboring nomads from trade and other interactions – and men who could ride. As cavalry warfare spread, the commercialized northern frontier, with horses to offer, became strategically important to its neighbors.
Both the Central States and non-Zhou tribes built long walls of packed earth. Closely hugging the high ground, the walls allowed soldiers to look far and wide for enemy movement. Large numbers of soldiers manned the walls, supported by garrisons (of which archaeologists have found evidence), roads and beacon stations, couriers and postal systems for communication. Contrary to the common view, the long walls did not mark a clear cultural boundary between steppe and sown (agricultural land), for there is plenty of steppe land, with nomadic sites, to the south of the walls, and some agriculture north of them. Nor were walls built to defend against invasions from the north. Yan, Zhao, and Qin built walls after they had expanded their states northwards, deep into nomad lands. The walls were forward offensive lines meant to hold new territory. Zhao, for instance, continued to expand beyond its wall, both as a matter of state policy and simply because soldiers raided for booty. Far from being effective defenses, walls enclosed new territory, and as territory grew, defense became more difficult.1