The First Era of Northern Domination refers to the period of Vietnamese history during which present-day northern Vietnam was under the rule of the Han dynasty and the interregnum Xin dynasty as Jiaozhi and Jiaozhou Commanderies. It is considered the first of four periods of Chinese rule over Vietnam, and the first of the three in which were almost continuous and was referred to as Bắc thuộc ("Northern Domination"). It began in 111 BC when the Western Han dynasty under its seventh emperor, Emperor Wu, conquered the defiant vassal state of Nanyue during its southward expansion and annexed all of the Lingnan region, which included what is today Guangdong and Guangxi together with much of modern northern Vietnam and half of central Vietnam. This era ended with the Trưng sisters' rebellion in 40 AD, which lasted three years before being quelled by Eastern Han dynasty general Ma Yuan, who restored Han rule and began Vietnam's Second Era of Northern Domination.
General info from Wikipedia.org