The Battle of Bch ng was the last major engagement during Mongol campaigns against Dai Viet and Champa, one of the greatest victories in Vietnamese military history. It was a naval battle between i Vit, commanded by Commander-in-Chief Prince Trn Quc Tun (Prince Hng o), and the invading army of the Yuan dynasty, commanded by Admirals Omar and Fan Yi on the Bch ng River (today Qung Ninh province), which Prince Hng o perfectly staged an ambush that destroyed the Yuan fleet, capturing its general, ending Kublais intention to conquer Dai Viet and Champa. The battle took place at the Bch ng River, near Ha Long Bay in present-day northern Vietnam. The battle was a tactical masterpiece of the same stature as a previous battle of Bch ng that occurred in 938.

The Mongol invasions of Vietnam were military campaigns launched by the Mongol Empire, and later the Yuan dynasty, against the kingdom of Đại Việt (modern-day northern Vietnam) ruled by the Trần dynasty and the kingdom of Champa (modern-day central Vietnam) in 1258, 1282–1284, 1285, and 1287–88. The campaigns are treated by a number of scholars as a success due to the establishment of tributary relations with Đại Việt despite the Mongols suffering major military defeats. In contrast, Vietnamese historiography regards the war as a major victory against the foreign invaders whom they called "the Mongol yokes."The first invasion began in 1258 under the united Mongol Empire, as it looked for alternative paths to invade the Song dynasty. The Mongol general Uriyangkhadai was successful in capturing the Vietnamese capital Thang Long (modern-day Hanoi) before turning north in 1259 to invade the Song dynasty in modern-day Guangxi as part of a coordinated Mongol attack with armies attacking in Sichuan under Möngke Khan and other Mongol armies attacking in modern-day Shandong and Henan. The first invasion also established tributary relations between the Vietnamese kingdom, formerly a Song dynasty tributary state, and the Yuan dynasty. In 1282, Kublai Khan and the Yuan dynasty launched a naval invasion of Champa that also resulted in the establishment of tributary relations.

Intending to demand greater tribute and direct Yuan oversight of local affairs in Đại Việt and Champa, the Yuan launched another invasion in 1285. The second invasion of Đại Việt failed to accomplish its goals, and the Yuan launched a third invasion in 1287 with the intent of replacing the uncooperative Đại Việt ruler Trần Nhân Tông with the defected Trần prince Trần Ích Tắc. By the end of the second and third invasions, which involved both initial successes and eventual major defeats for the Mongols, both Đại Việt and Champa decided to accept the nominal supremacy of the Yuan dynasty and became tributary states to avoid further conflict.