The answer is Operation Cedar Falls
Operation Cedar Falls was the largest American ground military operation of the Vietnam war, lasting for 18 days, from 8 to 26 January 1967. This offensive aimed to clear out strongholds of the National Liberation Front (also called Viet Cong) around Saigon. Its main targets were the Thanh Dien Forest Preserve and the "Iron Triangle", which was a 60-square-mile jungle located northwest Saigon that American forces believed that contained communist base camps and supply dumps.
