[3] Liberated after an amnesty, he returned to Sicily but volunteered again with Garibaldi's troops in the Third Italian War of Independence in 1866 and participated in the Battle of Bezzecca in Trentino, northern Italy, in July 1866.

In 1867, he returned to Castrogiovanni due to the death of his father but immediately left to join Garibaldi again in his new campaign to capture Rome.

He arrived too late when the Battle of Mentana, in which Garibaldi was defeated by Papal troops and a French auxiliary force, had already ended.

He remained in prison until 17 November, when an amnesty was declared because of the birth of the future king of Italy, Victor Emmanuel III.

[4][3] After graduating in Medicine in 1871, Colajanni enrolled as a physician on a ship to South America before returning to Italy to devote himself to the study of sociology and continue his political activities.

[4] With his book Il socialismo (Socialism) published in 1884 in Catania, Colajanni became one of the first theoreticians of the Italian workers movement.

His socialism was not based on the scientific Marxist approach but was closer to the ideology of Mazzini, one of the fathers of Italian unification, with some influence of French utopian thinkers such as Georges Sorel, and in terms of practical politics resulted in a kind of radical-democratic reformism.

Since 1896, he directed the Rivista popolare (Popular Magazine), by means of which he strove to improve the moral and intellectual standard of the masses and combated all forms of intolerance and hypocrisy.

[4][5] Colajanni published many books and essays on social and political problems, and exposed the unscientific theories of Cesare Lombroso and his Scuola positiva (Positive School),[6] as well as Enrico Ferri on criminology.

[5] Colajanni was particularly critical of Lombroso's biological determinism, in particular the alleged inferiority of Southern Italians, and he put a much greater emphasis on social conditions as a cause of offending.

[10] In his essay Per la razza maledetta (For the Cursed Race, published in 1898), Colajanni ridiculed the history of anthropometry and its related categories of the Lombrosian school and deconstructed their ethnic stereotypes.

He opposed the notion of racism and racial superiority as an ideological tool to legitimise dominance and exploitation, which would lead to the destruction of other races instead of its alleged progressive transformation.

Although poor health forced him to stay at Castrogiovanni, he continued to write political articles in periodicals of democratic orientation.

On 20 December 1892, Colajanni read out long extracts in Parliament and the then Historical Left prime minister Giovanni Giolitti was forced to appoint an expert commission to investigate the bank.

In the three weeks of uncertainty before the government was formed, the rapid spread of violence drove many local authorities to disregard Giolitti's ban on the use of firearms.

[19] Within a few days of the declaration of martial law and the violent suppression of the Fasci, Colajanni broke with Crispi and wrote the book Gli avvenimenti di Sicilia e le loro cause (The Events in Sicily and Their Causes) on the events in Sicily, which put the main blame on Crispi.

On the basis of dubious documents and reports, Crispi alleged that there was an organised conspiracy to separate Sicily from Italy, that the leaders of the Fasci conspired with the clerics and were financed by French gold, and war and invasion were looming.

[16][17] Disillusioned by the spread of violence in Sicily, to which he believed the PSI's discourse of class struggle had contributed, Colajanni reverted in 1894 to his original republicanism.

[20] A recurring theme of Colajanni's political engagement was the struggle to overcome the economic contrast between North and South of Italy, through a reform of society but also of the state through federalism.

[4] In 1900, Colajanni wrote a j'accuse directed at the magistracy, the police, and the government in relation to the trial about the 1893 murder of Emanuele Notarbartolo, the ex-mayor of Palermo and ex-governor of the Bank of Sicily.

Despite his anti-militarist ideas, he became an ardent supporter of the left-interventionist camp on the side of the Triple Entente at the outbreak of the First World War.