Toward the end of March 1951, he was drafted to join the Army and enlisted in the Đà Lạt Military Academy.
In April of the same year, he attended the Regiment Commander training at the Center for Military Studies in Sài Gòn.
In early 1957, after completing the training in the US, he was transferred to Thủ Đức Military Academy to hold the position of battalion commander of the 6th class of The Reserve Officer Cadet.
On 20 December 1963 he moved back to the administrative field to be the major of a newly re-established Gò Công province.
However, Civil Operations and Revolutionary Development Support (CORDS) advisers differed and in 1967 found "his personal cautiousness and reluctance to push the battalions [those in securing missions] into more offensive activities... difficult to understand," claiming that he discouraged the initiative and aggressiveness of his subordinates."
"[3]: 60 Brigadier General George Wear commented, "When the ARVN troops were well-led they fought as well as anyone's soldiers."
[3]: 61 On 2 May 1970, Thanh flew in a helicopter to direct the Cửu Long (Mekong) offensive operation over the Cambodian border to destroy communist sanctuaries.
He only revealed the plan to General Hal D. McCown, senior adviser of Delta Regional Assistance Command.
One day before the start of the Cửu Long (Mekong) operation, 1 May 1970, he instructed the 9th division commander on what he wanted him to execute.