In 1946, France attempted to reassert control over its colony and conventional military fighting broke out. Thảo served with the Vietminh in the Mekong Delta in the far south of Vietnam during the war against French rule from 1946 to 1954. He almost met his end before he had started; he was apprehended by the local communists in Mỹ Tho, who saw his French-style dress and mistook him for a colonial agent. They tied him up and chained him to a block of stone before throwing him into a river to drown. However, Thảo broke free of the weight and swam to safety. Thảo proceeded further south and deeper into the Mekong Delta to the town of Vĩnh Long, where he was again arrested by the local Vietminh. Just as Thảo was about to be executed by drowning, one of the communists realised he was a brother of one of their comrades. Thảo was released and rejoined his family, who lived in the region.