A member of the Roman Catholic minority, Phát joined the French-backed Vietnamese National Army which became the ARVN after the Republic of Vietnam (South Vietnam) was established. After having been sent to the U.S. for further training in 1958, Phát returned home to head the Civil Guard, a paramilitary force then mostly used to protect the ruling family of President Ngô Đình Diệm, rather than to counteract the communist Việt Cộng insurgency. Later commanding the 2nd Division, he was known for his loyalty to Diệm, who favored fellow Catholics. In 1963, Diệm knew some of the generals were about to launch a coup against him. He appointed Phát to command the 7th Division, located near the capital, Saigon, so he could help in the fighting, but the plotters successfully used delaying tactics so that the paperwork for the transfer of the division leadership could not occur before they proceeded to overthrow and execute Diệm. Despite Phát's pro-Diệm allegiance he was promoted to brigadier general and given command of the 7th Division.