 During the Mexican–American War, Nelson served as a captain in the Kennesaw Rangers with another future mayor, Cicero C. Hammock, as well as the father of mayor John B. Goodwin – Williamson H. Goodwin.Nelson later served as a brigadier general under General Narciso López, in a failed attempt to free Cuba from Spain.He organized and served as colonel of the 10th Texas Infantry Regiment in the Confederate States Army.