On October 7, 1571 the forces of the Holy League under Don Juan of Austria, illegitimate half brother of Philip II, in an ever-lasting tribute to Italian and Spanish courage and seamanship, smashed the Turkish fleet. This was the turning point in the centuries-long struggle between the Christian West and the forces of the Ottoman Empire over the Mediterranean. The Holy League had been the work of Pope Saint Pius V, who miraculously saw the victory in Rome on the day of the battle, and he proclaimed the feast day of Our Lady of Victory to whom he attributed the victory.
For a good overview of the battle of Lepanto read this review by Victor Davis Hanson here of The Victory of the West: The Great Christian-Muslim Clash at the Battle of Lepanto by Niccolò Capponi.
Before the battle Don John of Austria went about the ships of his fleet and said this to his crews: ‘My children, we are here to conquer or die. In death or in victory, you will win immortality.’ The chaplains of the fleet preached sermons on the theme: “No Heaven For Cowards”. Many of the men were clutching rosaries just before the battle. Admiral Andrea Doria went into the fight with an image of Our Lady of Guadalupe aboard his ship. Back in Europe countless Catholics were praying rosaries at the request of Saint Pope Pius V for the success of the Christian fleet.
At the hour of the battle, and this fact is very well attested, the Pope was talking to some cardinals in Rome. He abruptly ceased the conversation, opened a window and looked heavenward. He then turned to the cardinals and said: “It is not now a time to talk any more upon business; but to give thanks to God for the victory he has granted to the arms of the Christians.” So that Catholics would never forget Lepanto and the intercession of Mary, he instituted the feast of Our Lady of Victory. To aid in this remembrance G. K. Chesterton in 1911 wrote his epic poem Lepanto: