Lý Thường Kiệt
Relations between the kingdom of Đại Việt and the Song Empire began to deteriorate when the Song chancellor Wang Anshi brought out his reforms in 1069. Wang Anshi at the Chinese imperial court planned to reconquer “Jiaozhi” and forbade border prefectures from trading with it: “Then the king of Jiaozhi resolved to make war." King Lý Nhân Tông decided to stay one step ahead by ordering the generals Lý Thường Kiệt and Tôn Đản to launch a military campaign under the pretext of "rescuing Chinese people from the cruel reforms." In 1075, the Vietnamese army led by Lý Thường Kiệt attacked the Lian and Qin provinces of Song China while another army led by Tôn Đản besieged the citadel of Yongzhou. Although Lý Thường Kiệt succeeded in defeating the Song reinforcements intended for Yongzhou, the Song army and people in the city were still able to stand the besiegement for 40 days and inflicted high casualties on the Vietnamese forces. As a result, when the Vietnamese army finally ended the siege of Yongzhou, the Vietnamese generals decided to get revenge by killing more than 58,000 people in the city. Several sources estimate that the total number of people killed by Viet troops during this campaign totaled 100,000. In response to the sudden attack, the Song emperor began an invasion of Đại Việt in the twelfth month of 1076 with the support of the Song's vassal kingdoms, Champa and the Khmer. After an initial, rapid advance, the Song forces were stopped by the Vietnamese defense along the Như Nguyệt River led by Lý Thường Kiệt. In avoiding this strong defence system, the commander of the Song army decided to change the army's direction towards the nearby region of Phú Lương, where they defeated Lý Thường Kiệt's forces in a major battle. As the Song forces took the offensive, while Đại Việt had to strenuously hold on to their costly defensive front, Lý Thường Kiệt tried to boost the morale of his soldiers by a poem named "Nam quốc sơn hà". Afterwards this poem was considered the first Declarations of independence of Vietnam. Being aware of the unfavourable situation of the Viet army, Lý Nhân Tông finally decided to propose a cease-fire which was accepted by the Song dynasty since they had lost 400,000 men to battles and the harsh weather of Đại Việt. The Song army retreated after occupying a vast region near the border between the two countries; however, this region was gradually returned to the Đại Việt after several diplomatic efforts made by the Vietnamese envoys Đào Tôn Nguyên and Lê Văn Thịnh.