Badcoe arrived in South Vietnam on 6 August 1966 as a member of the AATTV. He was posted as a sub-sector adviser in the Nam Hóa District of Thừa Thiên Province. Sub-sector advisers worked at the district level with two elements of the Territorial Forces, the Regional Force (RF) and Popular Force (PF), which were both forms of full-time provincial militia under control of the provincial chief, who was also the military sector chief. Each sector corresponded with a province, and each sub-sector with a district. The sector and sub-sector advisers had several responsibilities: to accompany the RF/PF, colloquially known as "Ruff-Puffs", on operations; to provide on-the-job training to the RF/PF; to oversee security in the hamlets; and to liaise with the ARVN troops operating in the sector or sub-sector.