La forza del destino
From Royal Opera House website:
The Marquis of Calatrava forbids his daughter Leonora to marry the South American nobleman Don Alvaro. The lovers attempt to elope, but the Marquis catches them. In the ensuing altercation, Alvaro accidentally kills the Marquis, who curses his daughter as he dies. Leonora and Alvaro become separated during their escape. Leonora's brother Don Carlo di Vargas decides to find them and avenge his father.
Don Carlo joins the Spanish army under a false name, little knowing that Don Alvaro has done the same thing. The men become close friends while fighting in Italy. But friendship turns to enmity when Don Carlo discovers Leonora's portrait among Alvaro's possessions, and realizes who his new 'friend' really is. His subsequent actions lead the men back to Spain, and to the remote monastery where a disguised Leonora lives as a recluse.