Alphonsus VIII, King of Aragon and Castille, had been leading a disorderly life and had been punished by God in several ways, and he was forced to take refuge in a town belonging to one of his allies. St. Dominic happened to be in this town on Christmas Day and he preached on the Rosary as he usually did, and spoke of the graces that we obtain through this devotion. He mentioned, among other things, that those who said the Rosary devoutly would overcome their enemies and regain all they had lost.
The King listened attentively and sent for St. Dominic to ask whether what he had said about the Rosary was really true. The Saint assured him that nothing was more true, and that if only he would practice this devotion and join the Confraternity, he would see for himself. The King resolved to say the Rosary every day and persevered for a year in doing so. The very next Christmas, our Lady appeared to him at the end of his Rosary and said, “Alphonsus, you have served me for a year by saying my Rosary devoutly every day, so I have come to reward you. I have obtained the forgiveness of your sins from my Son. Here is a rosary, which I present to you; wear it, and I promise you that none of your enemies will be able to harm you.”
Our Lady vanished, leaving the King overjoyed and greatly encouraged; he immediately went in search of the Queen and told her all about our Lady’s gift and the promise that went with it. He touched her eyes with this
rosary, for she had lost her sight, and she was cured. Shortly afterwards the King rallied some troops and with the help of his allies boldly attacked his enemies. He forced them to give back the territory they had taken from him and make reparation for his losses. They were completely routed, and he became so successful in war that soldiers came from all sides to fight under his standard, because it seemed that, whenever he went into battle, the victory was sure to be his.
This is not surprising because he never went into battle without first saying his Rosary on his knees. He made certain that the whole of his court joined the Confraternity of the Rosary and he saw to it that all his officials and servants were devoted to it. The Queen also joined the Confraternity, and they both persevered in the service of Blessed Virgin and lived very holy lives.
Source: Miraculous Stories of the Rosary by Saint Louis de Montfort