In October 1571, off the Gulf of Patras, one of the most important battles in the history of the modern Mediterranean was fought, and the last battle to witness the use of galleys as attack ships: the Battle of Lepanto. The fleet of the Holy League, composed mainly of Venetian and Spanish forces, definitively defeated the Turkish fleet of the Ottoman Empire, dealing a decisive blow to its enemy. In this victory, the role of Venetian intelligence was extremely important and contributed decisively to the League's victory.

The Serenissima had set up an efficient system of spies in Istanbul, Alexandria, Smyrna and Algiers with the aim of collecting, on a daily basis, detailed information on the movement of the Ottoman fleet. Thanks to Venetian agents, the Serenissima was informed in time of Sultan Selim II's intentions to attack Cyprus, a Venetian stronghold in the eastern Mediterranean. The community of Venetian merchants, diplomats and dignitaries in Istanbul was considerable and Venice often used people recruited locally to carry out espionage missions. Ciphered messages and information bulletins were sent home hidden inside books, wine barrels or even concealed by means of coded messages inside trade ledgers.

Venetian agents managed to infiltrate the Ottoman bureaucratic apparatus itself, exploiting the discontent of the Janissaries, creating administrative and logistical confusion aimed at slowing down the supply and fitting-out of the Turkish fleet in the shipyards. In this dense network of informers, a central role was played by the Venetian Bailo at the Sultan's court. The Bailo was the Venetian ambassador and in addition to his diplomatic post he was also secretly appointed head of local intelligence. Another important role was that of the Dragomanni who worked for Bailo, i.e. interpreters skilled in the Turkish language and able to translate and transmit the contents of confidential documents to Venice.

The triumph of Lepanto was due to the contribution and efficiency of this complex espionage machine.