mountain pass in the Annamese Cordillera (Chaîne Annamitique) between northern Vietnam and Laos, 55 miles (90 km) northwest of Dong Hoi, Vietnam. The pass lies 1,371 feet (418 m) above sea level and carries the road from Tan Ap in Vietnam to Muang Khammouan (formerly called Thakhek) in Laos, on the Mekong River. The strategic pass was the principal point of entry of the Ho Chi Minh Trail access system into Laos and was heavily bombed during the Vietnam War (1955–75).