France dropped its army in a valley; the enemy hauled artillery up the mountains
In 1954, French paratroopers fortified Dien Bien Phu, a valley outpost near Laos, on the assumption that no heavy Vietnamese artillery could reach it through the surrounding mountains. Viet Minh forces instead hauled their guns up the peaks piece by piece, crippled the airstrip France depended on for supply, and after a 55-day siege forced the garrison to surrender. The defeat ended French rule in Indochina and led directly to the 1954 Geneva Accords partitioning Vietnam.