Coordinates: 21°50′52″N 106°45′28″E / 21.84778°N 106.75778°E / 21.84778; 106.75778

Lạng Sơn ( listen), sometimes Langson, is a city in far northern Vietnam, is the capital of Lạng Sơn province. It is accessible by road and rail from Hanoi, the Vietnamese capital, and it is the northernmost point on National Road 1A.

Due to its geography as Vietnam's gateway to China, Lạng Sơn and its ancient citadel have been in the path of many invasions, and were the site of three French defeats during the colonial era. Occupied by Qing forces during the military buildup that preceded the Sino-French War, the city was occupied by France after a two-week campaign in February 1885. However, the brigade there conducted a hasty retreat after a failed attack through Bang Bo into China; the "retreat from Lạng Sơn" became the most controversial aspect of the war and led to the fall of Jules Ferry's ministry. French colonial forces clashed with the Japanese 5th Division in the Battle of Lạng Sơn during the Japanese Vietnam Expedition in 22 September 1940. The French were again compelled to retreat hastily.^{[citation needed]}