The oldest documents in ancient Chinese literature such as Giao Chau Ngoai Vuc Ky, Quang Chau Ky... all recorded An Duong Vuong as "son of King Thuc" (Thuc Vuong Tu), but did not clearly state the origin of King Thuc, the location of Thuc country and the name of An Duong Vuong.
Our oldest historical book, Dai Viet Su Luoc, only records one sentence about the origin of An Duong Vuong: "At the end of the Zhou Dynasty, Hung Vuong was defeated and replaced by Phan, the son of the Thuc King." The author of this historical book added a new element: An Duong Vuong's name was Phan, but did not specify where he was from.
Since the 15th century, with Dai Viet Su Ky Toan Thu and Linh Nam Chich Quai, An Duong Vuong clearly appeared in history books. Dai Viet Su Ky Toan Thu opened a section "Ky Nha Thuc" and wrote: "family name Thuc, given name Phan, from Ba Thuc land". Most of the historical books of the Le and Nguyen dynasties wrote like that. From then on, An Duong Vuong Thuc Phan was considered the son of the Thuc king, a Ba Thuc person and the founding date of Au Lac country was determined to be the year Giap Thin, the 58th year of Chu Noan Vuong, or 257 BC. But by the 19th century, there began to be doubts or denials about the origin of An Duong Vuong as a "Ba Thuc land".
According to Chinese history books, the Shu Kingdom in Sichuan was one of the countries during the Spring and Autumn - Warring States period. That country was destroyed by the Qin Kingdom in the 5th year of King Zhou Shen (316 BC). The last Shu King, Shu Kaiming, was killed by the Qin army in Wuyang and the crown prince, the son of the Shu King, also died in Bailu Mountain. However, according to our official history, the Au Lac Kingdom was not established until 257 BC. So after the Shu Kingdom was destroyed, where did the "son of the Shu King" exist and how did he cross thousands of miles of mountains and forests, passing through the territory of many countries in the Southwest to attack the Hung King's Van Lang Kingdom from Ba Shu? All doubts and denials first came from there.
Nguyen Van Sieu (1795-1872) almost denied the orthodox theory, considering the old historical records about An Duong Vuong according to Quang Chau Ky as mistaken. The authors of the book Kham Dinh Viet Su Thong Giam Cuong Muc raised the question: "Thuc country, since the 5th year of Than Tinh Vuong of the Zhou Dynasty (316 BC) was destroyed by Qin, how could there still be a king? Moreover, from Thuc to Van Lang, there were also Kien Vi land, Da Lang land, Cung land, Tac land and Nhiem Mang land, separated by two or three thousand miles, could Thuc have passed through those countries, to attack and take Van Lang country?". Historians of the National History Institute of the Nguyen Dynasty cautiously raised a new noteworthy hypothesis: "Or perhaps outside the northwest bordering Van Lang country, there was another Thuc family that the old history recognized as Thuc Vuong?".
Under the French colonial period, the tendency of skepticism or denial grew. Vietnamese History, based on the Imperial Annals of Vietnam, argued that An Duong Vuong Thuc Phan "was not from the Thuc dynasty in China". Ngo Tat To analyzed the arguments more deeply, affirming that "the Nam nation did not have An Duong Vuong from the Thuc dynasty". Among the Western Orientalists who studied ancient Vietnamese history, H. Maspero also expressed doubts about the authenticity of An Duong Vuong because, according to the author, old history was only recorded according to legend. L. Aurousseau recognized the character of An Duong Vuong, but believed that An Duong Vuong's Au Lac nation only existed for 3 years from 210 to 208 BC.
Since the August Revolution, especially since the 1960s, research on the An Duong Vuong period has been considered a period of the first nation-building era, and has attracted more and more attention and achieved some new achievements. Regarding the origin of An Duong Vuong and the founding of Au Lac, there have been new researches and discoveries of documents, and on that basis, new hypotheses have been put forward.
In the 1950s, the traditional theory of An Duong Vuong's Ba Thuc origin was still defended by many historians, but with new explanations.
Some people believe that after the Shu kingdom was destroyed by the Qin army, the descendants of the Shu king from Ba Thuc land went south to hide, then gradually entered Viet land, established the Au Lac kingdom with the Thuc An Duong Vuong dynasty, which existed for about 5 years from 210 to 206 BC.
Some people believe that Thuc Phan could be the son or distant descendant of King Thuc in Ba Thuc. After the country was destroyed, he and his clan fled to Dien Tri, then followed the Red River to Lac Viet land, occupying Tay Vu in the northwest of the present-day midlands of Bac Bo. After leading the Lac Viet and Tay Au people to successfully resist the Qin army, Thuc Phan occupied Hung Vuong's Van Lang country and established Au Lac country around 208 BC.
Some people, based on the distribution of the population in the Southwest Di region, speculate that the King of Shu in ancient documents was not the king of Shu in Ba Shu, but the chief of the Jiang tribe who migrated from Shu to the south and called himself King of Shu. That Shu tribe went down to Guangxi and the Northern Delta to live together and assimilate with the Western Au people in this region. The Au Lac country included two groups of residents: Lac Viet and Tay Au.
In 1963, a new document about An Duong Vuong Thuc Phan was published. It was the legend "Cau chua cheng vua" (nine lords fighting for the king) of the Tay ethnic group in Cao Bang. According to this legend, Thuc Phan was the son of Thuc Che, "king" of "Nam Cuong country" in the present Cao Bang and Quang Tay regions, with the center being Hoa An (Cao Bang). Nam Cuong consisted of 9 Muong regions. At the end of the Hung King's reign, Thuc Che died, and his son Thuc Phan was still young. Nine Muong lords brought troops to capture Thuc Phan, divided the land into small parts for the lords to manage, and demanded to give up the "king" throne. Although Thuc Phan was young, he was very intelligent and organized competitions of strength and talent, and promised that whoever won would be given the throne. Thuc Phan used tricks to make the lords spend a lot of effort, but no one won. In the end, the lords had to submit to Thuc Phan. After that, "Nam Cuong country" became prosperous. Thuc Phan, taking advantage of the weakness of Van Lang country, invaded and established Au Lac country, calling himself An Duong Vuong, with the capital in Co Loa.
Based on the legend "Cầu chua cheng vua", a new hypothesis emerged, suggesting that Thuc Phan was the chief of a tribal alliance of Western Europeans or ancient Tay people in the mountainous and forested areas of northern Bac Bo and southern Quang Tay.
In the late 1960s, two new hypotheses appeared about the origin of Thuc Phan.

According to the Hung King's genealogy and many sacred stories, the genealogy of Saint Tan Vien and the generals of Tan Vien, Thuc Phan was the "father of Ai Lao land", the "lord of Ai Lao". Based on that data combined with the records of ancient Chinese books about the Southwest Di region, some people hypothesized: Thuc Phan was not the king of Thuc in Sichuan, but the king of Tay Thuc of the "Ai Lao di" in the Yunnan region, bordering Van Lang in the northwest. At the end of the Hung King's reign, Thuc Phan invaded Van Lang.
Some people re-analyzed ancient Chinese documents, proving that the Lac Viet people were distributed over a large area from the Red River basin to the Tay Giang basin. Thuc Phan was the leader of a certain Lac Viet group and Au Lac or Tay Au or Tay Au Lac was also a country of the Lac Viet people founded by An Duong Vuong. With this hypothesis, the author believes that An Duong Vuong's occupation of Van Lang country and establishment of Au Lac country was only an internal struggle of the Lac Viet people.
All of the above hypotheses prove that the origin of Thuc Phan and the birth of Au Lac still have many mysterious aspects that need to be further explored and researched. However, the general trend of researchers in recent times has been to reject the traditional theory of the origin of Ba Thuc of Thuc Phan. This theory not only contains absurdities and is difficult to explain about the distance in space and time between Thuc in Sichuan and Au Lac in northern Vietnam as stated in the Complete Annals of Vietnam, but also contradicts many domestic sources.

According to this theory, Thuc Phan was a foreigner, an invader of Van Lang country. But in the long-standing memories and feelings of our people reflected in the legends, jade genealogies, in worship rituals, folk performances... An Duong Vuong Thuc Phan was absolutely not an enemy, but a person who had contributed to building and defending the country, a revered hero. If March 10 (lunar calendar) is the death anniversary of Hung Vuong, January 6 (lunar calendar) is also a big festival at the Thuc King temple in Co Loa:
When you die, you leave your children and grandchildren behind. 
When you live, you don't leave the sixth day of the first lunar month behind.
In many legends about Hung Vuong, An Duong Vuong and other figures of that time, as well as folklore, Thuc Phan was a "lord", a "subordinate" belonging to the "Hung King's lineage". The conflict between Hung and Thuc did not lead to annexation and mutual destruction, but ended with the abdication of Hung Vuong and the unification of the people and land to establish Au Lac country. Therefore, a common direction established in recent hypotheses is to consider Thuc Phan as the leader of a tribe, a tribal alliance, a "country" near Van Lang country and having close relations with Van Lang residents.
Synthesizing the research results to date, we can tentatively establish the following hypothesis:
The inhabitants of Van Lang country during the Hung King period were mainly Lac Viet people and also included a group of Tay Au people (also known as Au Viet) in the mountainous and midland areas of the north. These two groups lived together in many areas. The north of Van Lang country was the area of residence of Tay Au people, and there were also Lac Viet groups living together. Many ancient Chinese sources prove that Lac Viet people were present in the Tay Giang basin, in the Guangdong, Guangxi, Hainan island and part of Yunnan and Guizhou.
Lac Viet and Tay Au are two southern groups of the Bach Viet, living close to each other and having intermingled areas, in the Red River and Tay Giang river basins. Being of the same race and neighbors, the Lac Viet and Tay Au people have long had many close economic and cultural relations. Perhaps due to the intermingling and close relations in such aspects, ancient Chinese documents sometimes distinguish Tay Au from Lac Viet, and sometimes consider Tay Au and Lac Viet as one. Quach Phac (Jin Dynasty) commented on the book Phuong Ngon by Duong Hung and said: "Tay Au is a separate race of Lac Viet". Nhan Su Co (Tang Dynasty) when commenting on the Han Shu, also explained: "Tay Au means Lac Viet, saying Tay [Au] is to distinguish it from Dong Au".
Thuc Phan was the leader of a Western European tribal alliance in the north of Van Lang country. According to the legend of the Tay people, that tribal alliance was "Nam Cuong country" consisting of 10 muong regions (9 muongs of 9 lords and 1 central muong of Thuc Phan), which means 10 tribes formed, with the residential area including the southern region of Quang Tay, Cao Bang and possibly wider, the mountainous region of the north of Bac Bo, with the center being Cao Bang. The people of Co Loa (Hanoi) also said that An Duong Vuong Thuc Phan was originally "a mountain chieftain", a native of the mountainous region of the north.
Between the Lac Viet and Tay Au, as well as between Hung and Thuc, there have been many long-standing relationships. The legend of Lac Long Quan-Au Co contains the relationship between the two elements of Lac and Au in the ancient origins of the ethnic groups in Vietnam. Many legends and myths about Hung Vuong and An Duong Vuong consider Thuc Phan to be of the "lineage", "sect", or "grandson" of Hung Vuong. The legends that record Thuc Phan as "Ai Lao lord" or "Ai Lao tutor" also consider him as the head of a "department" like the 15 departments of Van Lang, also belonging to the "lineage of Hung Vuong" (Hung Vuong jade genealogy), not a strange foreign country.
But on the other hand, at the end of the Hung King's reign, a long-lasting conflict arose between Hung and Thuc. Many villages in the central region of the Red River basin worshiped Saint Tan Vien and the generals of King Hung had followed Saint Tan Vien to "fight the Thuc invaders". These were inevitable conflicts in the process of gathering tribes and tribal alliances close to each other to form a State and expand the scope of control of that State.
As the conflict continued, the Van Lang nation, as well as the Lac Viet and Tay Au people, and all the Vietnamese groups in the Bach Viet bloc faced an extremely dangerous threat. That was the large-scale invasion of the Qin empire. It was this historical context that explained why the Hung - Thuc conflict ended with the abdication of the Hung King to Thuc Phan and the establishment of the Au Lac nation as a successive development of the Van Lang nation, a unification at a higher level, with a wider scope, of the Lac Viet and Tay Au people.

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