The following year, Tendean received intelligence training in Bogor and was subsequently assigned to the Army Central Intelligence Service (). He was sent to the front lines during operation Dwikora, Indonesia's confrontation with Malaysia, where he led a group of volunteers in several infiltrations into Malaysia performing intelligence gathering. On April 15, 1965, Tendean was promoted to First Lieutenant () and later, was assigned as a personal aide to General Abdul Haris Nasution.