Dien Bien Phu lies in Muong Thanh valley, a 20-km-long and 6-km-wide basin sometimes described as "heart-shaped". It is on the western edge of Dien Bien province, of which it is the capital, and is only a short distance from the border with Laos.
(http://en.wikipedia.org/wiki/Dien_Bien_Phu)