By 300 B.C.E. states were sending out armies of a few hundred thousand drafted foot soldiers, usually accompanied by horsemen. Adding to their effectiveness was the development of the crossbow around 350 B.C.E. The intricate bronze trigger of the crossbow allowed a foot soldier to shoot farther than could a horseman carrying a light bow. To defend against crossbows, soldiers began wearing armor and helmets. Most of the armor was made of leather strips tied with cords. Helmets were sometimes made of iron.

The introduction of cavalry in this period further reduced the need for a chariot-riding aristocracy. Shooting bows and arrows from horseback was first perfected by non-Chinese peoples to the north of China proper, who at that time were making the transition to a nomadic pastoral economy. The northern state of Jin developed its own cavalry to defend itself from the attacks of these horsemen. Once it started using cavalry against other Chinese states, they too had to master the new technology. From this time on, acquiring and pasturing horses was a key component of military preparedness.

Because of these developments, rulers wanted to increase their populations, to have more commoners to serve as foot soldiers and more craftsmen to supply more weapons. To increase agricultural output, they brought new land into cultivation, drained marshes, and dug irrigation channels. Rulers began surveying their land and taxing farmers. They wanted to undermine the power of lords over their subjects in order to get direct access to the peasants’ labor power. Serfdom thus gradually declined.

The development of infantry armies also created the need for a new type of general, and rulers became less willing to let men lead troops merely because of aristocratic birth. In The Art of War (453–403 B.C.E.), Sun Wu described the ideal general as a master of maneuver, illusion, and deception. He argued that heroism is a useless virtue that leads to needless deaths. Discipline, however, is essential, and he insisted that the entire army had to be trained to follow the orders of its commanders without questioning them.