What country was the strongest during the Spring and Autumn and Warring States Periods

The state of Qin was a vassal state established by the Huaxia (an ancient name for the Han Chinese) in northwest China during the Zhou Dynasty, and the Qin people were one of the Huaxia who moved westward. Their ancestors, the Ying clan, were the right-hand men of the Shang Dynasty in guarding the Western Rong during the Yin and Shang Dynasties, and were valued by the Shang Dynasty as its aristocracy. Later on, the clan was punished by the Western Zhou rulers for being involved in a rebellion instigated by Wu Geng, and was forced to move westward, where they were reduced to slavery.

During the reign of King Xiao Wang of Zhou, Qin Fei Zi, an ancestor of Qin, was granted vassal status by the king of Zhou for his work in raising horses. Since then, the Qin people have been raising horses for the Zhou royal family and fighting against the Western Rong on the Hundred Days' Border. After King Yi of Zhou, the Zhou royal family became increasingly weak and had to rely on the Qin to stabilize peace in the western frontier.

In 821 B.C., Duke Zhuang of Qin defeated the Western Rong, and was appointed by King Xuan of the Zhou Dynasty to the position of Grand Master of the Western Frontier, and was given the land of Dogqiu (in the area of Lixian County, southwest of Tianshui City, Gansu Province), which was formerly inhabited by the Dailuo clan. In 771 B.C., when King Yu of Zhou was attacked and killed by the Western Rong, Duke Xiang of Qin was rewarded by King Ping of Zhou for leading an army to save Zhou.

In 770 B.C., Duke Xiang of Qin sent troops to escort King Ping of Zhou to move eastward, and was made a vassal and given the land west of Mount Qishan. Since then, Qin officially became a vassal state of the Zhou Dynasty.

At first, Qin was not taken seriously by the other vassal states because of its remote location. It was not until Duke Mu of Qin destroyed 12 states founded by the western Rong tribe, opened up more than 1,000 miles of land, and stabilized the rear of the country, that the foundation of the Four Great Powers of the Spring and Autumn Period was laid. Many of the Qin kings died fighting the Western Rong, and the Qin people's ability to fight the Rongs made them a warlike people.

At the beginning of the Warring States period, Wei attacked Qin for many years and seized the land west of the river, forcing Qin to retreat west of the Luo River. When Duke Xiaodong of Qin appointed Shang Yang to change the law, the state of Qin became stronger with each passing day, and gradually became the most powerful state in the mid-to-late Warring States period.

King Huiwen of Qin was proclaimed king in 325 BC. In 316 B.C., Qin annihilated the state of Shu, and from then on, Qin officially became the largest state among the Seven Kingdoms of the Warring States period.

In 246 BC, King Ying Zheng of Qin assumed the throne and established the Qin Dynasty by destroying the Six Kingdoms in the ten years between 230 and 221 BC.