After graduating from Đà Lạt, he soon saw action in a 1955 operation to eliminate the Bình Xuyên river pirates who were vying with President Diệm's government for control of Saigon and the surrounding area. In recognition of his performance against the Bình Xuyên, Trưởng was promoted to first lieutenant. When the Republic of Vietnam was created in 1955, the VNA became the Army of the Republic of Vietnam (ARVN). Rising through the ranks In 1964, Trưởng was promoted to major and made commander of the 5th Airborne Battalion. He led a heliborne assault into Đỗ Xá Secret Zone in Minh Long District, Quảng Ngãi Province, in central Vietnam. This attack destroyed the base area of the Việt Cộng's (VC) B-1 Front Headquarters. In 1965, Trưởng led the 5th Airborne Battalion on a helicopter assault into the Hát Dịch Secret Zone in the vicinity of the Ong Trinh Mountain in Phước Tuy Province southeast of Saigon, the base area of the VC 7th Division. In two days of fighting, Trưởng's 5th Battalion inflicted heavy casualties on two VC regiments, and he was awarded a battlefield promotion to lieutenant colonel and the National Defense Medal, Fourth Class. After the battle, Trưởng became chief of staff of the Airborne Brigade and then became chief of staff of the division later in the year. His reputation for valour and fairness gained the attention of the senior generals in Saigon. General Cao Văn Viên, chief of the Joint General Staff from 1965 to 1975, later described Trưởng as "one of the best commanders at every echelon the Airborne Division ever had." In 1966, the Buddhist Uprising broke out in central Vietnam with Buddhists protested military control of the government. Trưởng was asked to quell the rebellious 1st Division in Huế, which had decided to stop military operations against the VC in solidarity with the Buddhist protest movement. A Buddhist, Trưởng, was uncomfortable with his mission, but he carried out his orders. On 18 June, he commanded three airborne battalions that entered the city and restored order within two days and he put the 1st Division under government control. General As a result of his efficient display, Saigon made Trưởng's appointment as 1st Division commander permanent. With his hands-on leadership, Trưởng quickly moulded the unit, which had a poor reputation prior to his arrival, and had been heavily weakened by the infighting of the past