The A Shau Valley is a rugged, remote passageway near the border of Laos and the Ho Chi Ming Trail in Thua Thien province. It runs north and south for twenty-five miles. It's low, mile-wide, flat bottomland is covered with tall elephant grass and flanked by two strings of densely forested mountains that vary from three to six thousand feet. Because of its forbidden terrain and remoteness - and the fact it was usually hidden from the air by thick canopy jungle and fog and clouds - it was a key...