The birth of the Au Lac state was closely linked to the resistance war against the Qin and the role of Thuc Phan.
Before the resistance war against Qin, the Tay Au tribal alliance of Thuc Phan had many close relationships with the Van Lang country of Hung Vuong. The relationship of both exchange, association, struggle and conflict between the two ethnic groups that were close to each other in blood, territory, economy and culture was the basis and also the preparation for the unification of the Lac Viet - Tay Au ethnic groups to expand and develop the Van Lang country. The resistance war against Qin further promoted that development trend.
When entering our country, the Qin army first invaded the residential areas of the Western Au people. Thuc Phan, as the leader of the Western Au tribal alliance, of course had to organize and direct the resistance against the Qin. The further the Qin army advanced south, the more fierce resistance they encountered from the Western Au and Lac Viet people. The ancient Vietnamese leaders, due to the requirements of the war, promoted Thuc Phan to the highest commander. Perhaps that was the meaning of "they appointed the most talented people as generals" as Liu An wrote in Hoai Nam Tu.
The war that lasted for 5 or 6 years tightened the inherent solidarity between the Tay Au and Lac Viet people. The glorious victory of the resistance war further consolidated and enhanced Thuc Phan's prestige not only among the Tay Au but also among the Lac Viet people.
All the situations that took place before and during the resistance war against Qin were preparations for the establishment of Au Lac country to replace Van Lang country and transfer the throne from Hung Vuong to An Duong Vuong Thuc Phan.
During the resistance war, in fact, the Lac Viet - Tay Au community was formed. But Thuc Phan replaced Hung Vuong, proclaimed himself An Duong Vuong and established Au Lac country, probably after the successful resistance war. Dai Viet Su Luoc recorded that: "Hung Vuong was defeated and replaced by King Thuc's son Phan". Some legends and folk tales say that after many conflicts, Hung Vuong finally, following the advice of his son-in-law, Saint Tan Vien, abdicated to Thuc Phan.

The name of Au Lac country consists of two elements Au (Tay Au, Au Viet) and Lac (Lac Viet), reflecting the union of the two groups of Lac Viet and Tay Au people. In the Thuc king's dynasty, there were still Lac Hou and the localities were still governed by Lac Tuong. The territory of Au Lac country was also expanded on the basis of the two territories of Van Lang and Tay Au. The establishment of Au Lac country was not the result of a war of annexation and destruction, but a unification of the residents and land of Lac Viet and Tay Au, of Hung and Thuc. Therefore, Au Lac country was a new step of development, succeeding and surpassing Van Lang country. Therefore, not only Tay Au people, but also Lac Viet people and their descendants considered An Duong Vuong Thuc Phan a hero with great contributions to the cause of building and defending the country.
The Au Lac nation only existed for a short time, according to the above hypothesis, about 30 years (about 208-179 BC). Therefore, the existence of the Au Lac nation is not separated into a separate historical period, but is considered a continued development stage of the Van Lang nation and also lies within a common era: the era of nation building under the Hung Kings (Van Lang nation) and An Duong Vuong (Au Lac nation).
The economy, society, politics, and culture of the Au Lac period continued to develop on the basis of the achievements of the Van Lang nation. Dong Son culture remained the common cultural foundation of the Van Lang and Au Lac nations, but there were new developments in some aspects. Especially due to the urgent need to fight against foreign invaders, during the Au Lac period, military technology made remarkable progress.
An important weapon innovation of this period was the invention of the crossbow that could shoot multiple arrows at once, commonly known as the willow crossbow or the chain crossbow. This powerful new weapon was deified as the "god crossbow" (god of crossbows) in folk legends. In 1959, archaeologists discovered in Cau Vuc, south of Co Loa citadel, a few hundred meters from the outer citadel, a warehouse of tens of thousands of bronze arrows. It was a type of arrow with a three-sided arrowhead and a long handle inserted into the arrow. Arrows of different shapes were also found scattered in many places inside and outside the citadel. In 2024-2027, archaeological excavations at Thuong Temple discovered a system of bronze casting furnaces (with molds for casting three-sided bronze arrowheads, spearheads and waste from the furnace such as: bronze slag, arrowheads, mold sketches, raw stones, etc.).
A great work of labor, a famous military architecture that concentratedly demonstrated many development indicators of Au Lac country was Co Loa citadel.

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