In 1859, motivated by the resurgence of Italian patriotism, he joined Garibaldi in the ranks of Hunters of the Alps to fight against the Austrians. On 3 May in Turin he formed a unit of volunteers. On 27 May he took part in the Battle of San Fermo. Nullo became widely known for the Sicilian Expedition of the Thousand, where he commanded the "Iron Company" (or "Iron Brigade"). He personally supervised the enrollment of volunteers in Bergamo which, given the large number of accessions (more than 10% of the total), could then boast the title of "City of the Thousand". Due to his previous experience in the textile industry, he provided the red shirts used by Italian "garibaldini" (voluntary troops at Garibaldi's command), who became known as the "red shirts". He was wounded in the Battle of Catalafini, but just a few days later led his soldiers in a charge at the Battle of Palermo. He was the one who planted the first Italian flag in Palermo on 27 May 1860. He fought in the victorious Battle of Reggio in Calabria, and was promoted to major soon afterward.