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 Moreinu (our teacher). He was considered the greatest authority of his time, and communities far and wide sought his advice.
The Maharil lived during the period of the Hussite wars, which brought misery upon the Jews of the Rhine, Thuringia and Bavaria. The Jews appealed to the Gaon to intercede with G-d for them. The mercenary soldiers entered the Jewish homes at will and took everything they could lay their hands on. Should anyone protest, they would be shot on the spot.
When the Gaon received their desperate appeal, he sent messengers to the neighboring communities urging them to institute a general season of fasting and prayer. The German communities, obeying the call, fasted for seven days (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 Gaon 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 and many of them became Jews.
This was a dramatic episode on the immediate results of the power of repentance and prayer. (Jew, Ency. Mollin; G. Pollak Halikot Kedem pg. 79; Gratz, Gesch, 2nd ed. 8:136; Zunz, S.P. pg. 48).