The Âu Việt (Hán tự: 甌越), also known as Tây Âu (Xi'ou) kingdom in the Zhuang people's legend, was a conglomeration of upland tribes living in what is today the mountainous regions of northernmost Vietnam, western Guangdong, and northern Guangxi, China, since at least the 3rd century BCE. In the legends of the Tay people, the western part of Tây Âu/Âu Việt's land became the Nam Cương Kingdom, whose capital was located in what is today the Cao Bằng Province of Northeast Vietnam.
The Âu Việt were also referred to as the Kingdom of Eastern Ou (東甌), descendants of the state of Yue who had moved to Fujian after its fall. The Western Ou (西甌; Chinese: Xī Ōu; Tây meaning "western") were Baiyue tribes, with short hair and tattoos, who blackened their teeth and are the ancestors of the upland Tai-speaking minority groups in Vietnam such as the Nùng and Tay, as well as the closely related Zhuang people of Guangxi.
The Âu Việt traded with the Lạc Việt, the inhabitants of the state of Văn Lang, located in the lowland plains to Âu Việt's south, in what is today the Red River Delta of northern Vietnam, until 258 BC or 257 BC, when Thục Phán, the leader of an alliance of Âu Việt tribes, invaded Văn Lang and defeated the last Hùng king. He named the new nation "Âu Lạc", proclaiming himself "An Dương Vương" ("King An Dương").
The Qin dynasty conquered the state of Chu, unifying China. Qin abolished the noble status of the royal descendants of the state of Yue. After some years, Qin Shi Huang sent an army of 500,000 to conquer the West Ou, begain a three-year guerrilla war and killed their leader.
Before the Han dynasty, the East and West Ou regained independence. The Eastern Ou was attacked by the Minyue, and Emperor Wu of Han allowed them to move to between the Yangtze and the Huai River. The Western Ou paid tribute to Nanyue until it was conquered by the Han. Descendants of these kings later lost their royal status. Ou (區), Ou (歐) and Ouyang (歐陽) remain as family names.
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- Huainanzi 卷18, 人間訓