The feast of Our Lady of the Rosary has its origin not in simply a prayer, but in a battle.
On October 7, 1571, a fleet of ships assembled by the combined forces of the Holy League (Spain, Naples, Venice, the Papacy, Genoa, and others) fought an intense battle with the fleet of the Ottoman Empire, and their victory prevented the Turks from controlling the Mediterranean. One could say that the world that we know came into being with this victory at the Battle of Lepanto.
A victory was so important that Pope St. Pius V had encouraged the faithful to petition the intercession of the Blessed Virgin Mary through the recitation of the Rosary. When word reached the Pope of the victory of the Holy League, he added a new feast day to the Liturgical Calendar- October 7th would henceforth be the feast of Our Lady of Victory, later changed by his successor to Our Lady of the Rosary.
While we may feel uncomfortable associating the Blessed Mother with a battle, Pope St. Pius V interpreted the event as the movement of Providence in favor of the Church. Nowadays we celebrate the prayer of the Rosary, not the battle of Our Lady of Victory. And yet, while there is properly a peace making quality to our faith, there is also a properly oppositional quality to it; the recognition that in knowing what we stand for, we also know what we stand against. We have been placed in the midst of a fallen world, not merely to surrender to its failure to love, but to live in defiance to that which opposes God. In the midst of all our battles, Christ the Lord fights for us still, and our Lady of Victory is at our side.
Fr. Steve Grunow, Our Lady of the Rosary and the Battle of Lepanto
Excerpted from wordonfire.org, October 6, 2017