The economy is underdeveloped, the population is small, and the army is even more war-weary.

It should be;

During the Spring and Autumn Period and the Warring States Period, the productive forces developed rapidly, and the traditional farmland ownership system began to collapse. With the continuous expansion of the economic strength of the vassal States, the status of the Emperor of Zhou declined day by day. In order to obtain more land, population and taxes, the vassal States all fought for hegemony under the banner of respecting the king and resisting foreign countries.

In short, the fundamental condition is the development of productive forces, that is, economic development.