That Lady in Ermine (1948)

Circa 1861, Angelina, ruling countess of an Italian principality, is at a loss when invaded by the Hungarian army. Her lookalike ancestress Francesca, who saved a similar situation 300 years before, comes to life from a portrait to help her descendant. Complicating factor: the newlywed countess feels strangely drawn to the handsome invader.