Historically, the mainland and Vietnam are inextricably linked.
Dating back to 221 BC, after Qin Shi Huang established the Great Qin Empire, he continued to expand his territory and conquered hundreds of yue in the south, with remarkable results, occupying most of Lingnan. To facilitate his jurisdiction, he established three counties in Lingnan, of which Xiang County included parts of present-day Yunnan and Guizhou, as well as northern Vietnam.
In the Han Dynasty, the imperial court specially carved out the Vietnamese area and renamed it Jiaojian County. From then on, the area was under the rule of the Central Plains Dynasty until the end of the Tang Dynasty.
At the end of the Tang Dynasty, Zhuge divided one side, the central government was erected, peasant uprisings in various places continued, and the once prosperous Tang Dynasty came to an end in a blink of an eye, and China entered the period of five dynasties and ten kingdoms.
Taking advantage of the chaos, Vietnam's local warlord Ngo Quan proclaimed himself king in 939 AD, and since then, Vietnam has been free from Chinese rule.
Decades later, the Northern Song Dynasty was established, but the surrounding situation was tight.
It was necessary to deal with Liaojin and Mongolia, but also to face the invasion of Northwest and Western Xia, and the situation was grim. The Northern Song Dynasty itself was overwhelmed, and naturally had no extra strength to recover the Vietnamese region.
It was not until the Ming Dynasty that the Central Plains regained its strength again, and its military strength increased unprecedentedly. Ming Cheng Zu Zhu Di took advantage of the coup d'état in Vietnam, and immediately summoned 500,000 troops to march south, all the way to the ground, and it took less than a year to collect the entire territory of Vietnam.
However, it is easy to defeat, but it is difficult to establish an effective order, knowing that at that time, Vietnam had been independent for hundreds of years and had long formed its own national beliefs and culture. As a result, it was not long before the Vietnamese who disobeyed the rule rebelled, and Zhu Di had to deploy troops to suppress it.
In this way, the turbulent situation continued until Emperor Xuanzong of Ming ascended the throne.
Considering that the treasury was empty and the soldiers were exhausted due to the annual conquest, Emperor Ming Xuanzong decided to implement a foreign policy of comprehensive contraction and chose to withdraw his troops. In this way, the Ming Dynasty's 21-year occupation of Vietnam came to an end.
Then to the Qing Dynasty, when Qianlong also launched the Battle of Annam, and the defeated Vietnam voluntarily surrendered and became a vassal state, but the Vietnam of peace still maintained political and military independence, but only recognized the suzerainty of the Qing Dynasty and paid tribute regularly.
After the defeat of the Sino-French War in 1884, the Qing government abandoned its rule over Vietnam, and Vietnam became a French colony.
Our current territorial division is mainly based on the Qing Dynasty, and Vietnam was separated from the mainland long before the Song Dynasty, so we rarely mention that Vietnam was once China's territory.
Today, Vietnam and China are two complete countries.