The Blessed Virgin, the Holy Rosary and St. Dominic
"The well-known origin of the Rosary, illustrated in celebrated monuments of which we have made frequent mention, bears witness to its remarkable efficacy. For, in the days when the Albigensian sect, posing as the champion of pure faith and morals, but in reality introducing the worst kind of anarchy and corruption, brought many a nation to its utter ruin, the Church fought against it and the other infamous factions associated with it, not with troops and arms, but chiefly with the power of the most holy Rosary, the devotion which the Mother of God taught to our Father Dominic in order that he might propagate it. By this means the Church triumphed magnificently over every obstacle and provided for the salvation of her children not only in that trial but in others like it afterward, always with the same glorious success. For this reason, now, when human affairs have taken the course which We deplore, bringing affection to the Church and ruin to the State, all of us have the duty to unite our voice in prayer, with like devotion, to the holy Mother of God, beseeching her that we too may rejoice, as we ardently desire, in experiencing the same power of her Rosary."
This is paragraph 8 of the encyclical Magnae Dei Matris of the Holy Father Leo XIII, given on the 8th day of September, 1892, on the subject of the Holy Rosary, to which the Pope was most devoted. The whole of the letter may be found here.