The A Shau Valley is in the mountainous northwest corner of what used to be South Vietnam.  It is right across the border from Laos.  Because of its location, the A Shau formed a natural hub for the North Vietnamese Army to move their units and supplies into South Vietnam and threaten the more heavily populated coastal plains.  The United States tried multiple times and multiple ways to disrupt what the North Vietnamese were doing.  Some of the Vietnam War's heaviest combat ensued.  Jay Phillips tells the story in "A Shau: Crucible of the Vietnam War"