At Cao Bang you join Highway 4, an ambitious road that was originally part of a French military network linking the isolated garrisons right across northern Vietnam’s empty mountain country. This road is more subject than many others in the north to falling into disrepair, and the 140km journey to Lang Son takes about four hours, depending on conditions. Beyond Dong Khe, a nondescript town roughly 40km out of Cao Bang, the virtually traffic-free road climbs through a gorge of sheer limestone cliffs before cresting the dramatic Dong Khe Pass. In 1950 this pass was the scene of a daring ambush in which the Viet Minh gained their first major victory over the French Expeditionary Force. In the ensuing panic, forts all along the border were abandoned, an estimated six thousand French troops were killed or captured, and the Viet Minh netted 950 machine guns, eight thousand rifles and a few hundred trucks.
That Khe lies exactly halfway between Cao Bang and Lang Son, and beyond here the road winds through Na Sam, an attractive town snuggled beneath a dramatic setting of outcrops. The villages in this area are inhabited by Nung and Tay; their bamboo rafts and huge wooden waterwheels, which form part of sophisticated irrigation works, grace the river that weaves along beside the road. Unfortunately this rural idyll comes to an abrupt end, as the speedy Highway 1 from Dong Dang brings you hurtling into Lang Son.