Bride of Vengeance (1949)

The tiny independent duchy of Ferrara is located between Casare Borgia's Rome and Venice, and Borgia has plans to conquer Venice via Ferrara. He murders his sister's husband and makes it appear that Alfonso D'Este of Ferrara was behind the killing. To avenge herself against Ferrara and D'Este, Lucretia Borgia marries D'Este and intends to poison him. But...she falls in love with him.