A wonderful story is recorded about the Maharil, Rav Yaakov ben Moshe Mollin, who proved the power of repentance.
The Maharil was the first rav to bear the title Morenu. He was considered the greatest authority of his time, and communities far and wide sought his advice.
The Maharil lived during the Hussite Wars which brought misery upon the Jews of the Rhine, Thuringia and Bavaria. The Jews appealed to the Maharil to intercede with G-d for them. Mercenary soldiers would enter Jewish homes at will and take everything they could lay their hands on. Anyone who protested was shot.
When the Maharil received their desperate appeal, he sent messengers to the neighboring communities urging them to institute a period of fasting and prayer. Obeying the call, the German communities fasted for seven days (in September 1241).
The day following the fast, the grand duke, who had mobilized the large army, was suddenly stricken and died. Soon afterwards, the Imperial army and the mercenaries who were mobilized at Saaz dispersed and the very soldiers who had threatened and molested the Jews now came to them to beg for bread. Under the orders of the Maharil they were given bread and lodging. When they departed, they blessed the Jews and their G-d for treating them so kindly after having suffered so much at their hands, and many of them became Jews.