An excerpt from The Secret of the Rosary by St. Louis de Montfort. 

Public prayer is far more powerful than private prayer to appease the anger of God and call down His Mercy and Holy Mother Church, guided by the Holy Ghost, has always advocated public prayer in times of public tragedy and suffering.

In his bull on the Rosary, Pope Gregory XIII says very clearly that we must believe (on pious faith) that the public prayers and processions of members of the Confraternity of the Holy Rosary were largerly responsible for the great victory over the Turkish Navy at Lepanto which Almighty God granted to Christians on the first Sunday of October, 1571.  

When King Louis the Just, of blessed memory, was beseiging La Rochelle, where the revolutionary heretics had their stronghold, he wrote to his mother to beg her to have public prayers offered for a victorious outcome.  The Queen-Mother decided to have the Rosary recited publicly in Paris in the Dominican Church of Faubourg Saint Honore and this was done by the Archbishop of Paris.  It was begun on May 20th, 1628.  

Both the Queen-Mother and the reigning Queen attended the recitation of the Rosary together with the Duke of Orleans, Cardinal de La Rochefoucault and Cardinal de Berulle, as well as other prelates.  The court turned out in full force as well as a large proportion of the general populace.  The Archbishop used to read the meditations on the mysteries aloud and then being the Our Fathers and Hail Marys of each decade wihile the congregation made up of religious and lay folk answerd him.  At the end of the Rosary a statue of the Blessed Mother was solemnly carried in procession while the Litany of Our Lady was sung.  

This devotion was kept up with admirable fervor every Saturday and resulted in a manifest blessing from Heaven: for on All Saints' Day of the same year the king defeated the English at the island of Re and me his triumphant entry into La Rochelle. This goes to show the great power of public prayer.