Nanzhao was in alliance with local tribal rebels, against the Tang dynasty who was currently in control of the Red River Delta in modern-day northern Vietnam.

The siege took place in Songping (modern-day Hanoi), capital of Tang's frontier Protectorate General to Pacify the South in early 863 during the reign of Emperor Yizong.

It was the fourth time since 858 that Songping was attacked by Nanzhao forces The siege was one of the most important and tragic events in the history of Vietnam before the tenth century.

With 50,000 men from its main army and other tribal mercenaries combined, the Yunnanese approached Songping and issued an ultimatum to surrender or die.

They laid siege on the city from mid-January until its fall on 1 March, resulting in military disaster and the retreat of Tang forces out of the region in 2 years.

[3] The Annan Protectorate (now northern Vietnam), with its capital city of Songping, was a center of commerce on the Maritime Silk Road and a rice basket of the Tang dynasty at the time.

When Li Zhuo became jiedushi of Annan in 854, he reduced the amount of salt traded to the Chongmo Man in Fengzhou (modern-day Phú Thọ and Hòa Bình Province) in the west in exchange for horses.

[4] In the next year, Li Zhuo killed Đỗ Tồn Thành as well as the chieftain of the Qidong Man in Aizhou (Nghệ An, central Vietnam).

Fan Chuo, a Tang official in Annan reported: "…The native chiefs within the frontiers were subsequently seduced by the Man rebels…"[5] and "again became close friends with them.

"[5] In modern-day Phú Thọ and Hòa Bình Province on the western frontier of the protectorate, the local general Lý Do Độc who led an army of 6,000, and was assisted by seven commanders called "Lords of the Ravines", submitted to Nanzhao.

One evening they surrounded Songping and demanded that Wang Shi return north and allow them to fortify the city against the Yellow Head Army.

[9] Anti-Tang Viets allied with highland people, who appealed to Nanzhao for help, and as a result invaded the area in 860, briefly taking Songping before being driven out by a Tang army the next year.

When Li Hu led an army to retake Bozhou, the Đỗ family gathered 30,000 men, including contingents from Nanzhao to attack the Tang.

[14] Much of the information about the battle was written by Fan Chuo, a Tang official who wrote an eyewitness account about the southern barbarians (people of Annan and Yunnan) during the siege.

[15] In mid-January of 863, Nanzhao returned with an invasion force numbering 50,000 led by Duan Qiuqian and Yang Sijin and besieged Annan's capital Songping.

[19] On 28 January, a naked Buddhist monk, possibly Indian, was wounded in the breast by an arrow shot by Cai Xi while strutting to and fro outside the southern walls.

A Nanzhao and rebel fleet of 4,000 men led by a chieftain named Chu Đạo Cổ (Zhu Daogu, 朱道古) was attacked by a local commander, who rammed their vessels and sank 30 boats, drowning them.

After his recall, he was later reinstated and completed the retaking of Songping in fall 866, executing the enemy general, Duan Qiuqian, and beheading 30,000 of his men.