Dagmar and Nicholas
The father of princess Dagmar (1847-1928) was the king of Denmark. In 1864 she became engaged to the crown prince of Russia, Nicholas. Nicholas knew her by the cartes-de-visité he had seen of her, he always carried with him a photograph of Dagmar. When they finally met in 1864 they fell in love. When he suddenly became ill in 1865, and eventually died, she married his brother, the new crown prince Alexander. As Maria Feodorovna she became emperess of Russia in 1881.
Dagmar and Alexander
Another royal passed on fiancee: Princess Mary of Teck.