Giulio, afterwards Cardinal, Acquaviva had been sent at the end of 1568 to Philip II by the Pope on a mission, partly of condolence, partly political, and on his return to Rome, which was somewhat brusquely expedited by the King, he took Cervantes with him as his camarero (chamberlain), the office he himself held in the Pope's household.

The post would no doubt have led to advancement at the Papal Court had Cervantes retained it, but in the summer of 1570 he resigned it and enlisted as a private soldier in Captain Diego Urbina's company, belonging to Don Miguel de Moncada's regiment, but at that time forming a part of the command of Marc Antony Colonna.

What impelled him to this step we know not, whether it was distaste for the career before him, or purely military enthusiasm.

It may well have been the latter, for it was a stirring time; the events, however, which led to the alliance between Spain, Venice, and the Pope, against the common enemy, the Porte, and to the victory of the combined fleets at Lepanto, belong rather to the history of Europe than to the life of Cervantes.

He was one of those that sailed from Messina, in September 1571, under the command of Don John of Austria; but on the morning of the 7th of October, when the Turkish fleet was sighted, he was lying below ill with fever.