The northern 5 Provinces of South Vietnam made up I Corps. The Demilitarized Zone formed its northern border and Laos was to its west. The A Shau Valley in western I Corps was a major terminus of the Ho Chi Minh Trail and a logistical staging area for the North Vietnamese. The major cities in this area were Quang Tri, Hue, Danang, Tam Ky and Quang Ngai. Throughout most of the war I Corps was the responsibility of the III Marine Amphibious Force, the 101st Airborne, and the Americal Divisions. Unlike the rest of Vietnam, from the outset the war in I Corps was essentially against North Vietnamese regulars, organized as regiments and divisions, and suported by artillery. More than half of all US casualties occurred in I Corps.