By the 3rd century BCE, another Viet group, the Âu Việt, emigrated from present-day southern China to the Hồng River delta and mixed with the indigenous Văn Lang population. In 257 BCE, a new kingdom, Âu Lạc, emerged as the union of the Âu Việt and the Lạc Việt, with Thục Phán proclaiming himself "An Dương Vương" ("King An Dương"). Some modern Vietnamese believe that Thục Phán came upon the Âu Việt territory (modern-day northernmost Vietnam, western Guangdong, and southern Guangxi province, with its capital in what is today Cao Bằng Province).^[29]

After assembling an army, he defeated and overthrew the eighteenth dynasty of the Hùng kings, around 258 BCE. He then renamed his newly acquired state from Văn Lang to Âu Lạc and established the new capital at Phong Khê in the present-day Phú Thọ town in northern Vietnam, where he tried to build the Cổ Loa Citadel (Cổ Loa Thành), the spiral fortress approximately ten miles north of that new capital. Cổ Loa, the largest prehistoric moated urban settlement in Southeast Asia,^[30] was the first political hub of Vietnamese civilization in the pre-Sinitic era, encompassing 600 hectares (1,500 acres), and requiring as many as 2 million cubic meters of material. However, records showed that espionage resulted in the downfall of An Dương Vương.