He was a participant in the 1963 coup that deposed President Ngô Đình Diệm and ended in his assassination. Colonel Có was the deputy of General Tôn Thất Đính, who commanded the III Corps forces that oversaw the region surrounding the capital Saigon. Đính was entrusted to command III Corps because the Ngô family trusted him to defend them in the face of any coup attempts. However, in late 1963, Đính began to plot against Diệm along with a group of generals.