Royal Marriage / Alliance	Key Figures & Kinship	Geopolitical & Dynastic Impact
Philip II & Anna of Austria (1570)	Uncle and niece; Anna was the daughter of Philip's sister Maria.	Produced Philip III, cementing Habsburg control over Portugal and Spain while intensifying generational inbreeding.
Philip IV & Mariana of Austria (1649)	Uncle and niece; Mariana was Philip's biological sister's daughter.	Resulted in the birth of Charles II; led directly to the end of Habsburg rule and the 1701, 1714 War of the Spanish Succession.
Alfonso XIII & Victoria Eugenie (1906)	Princess Victoria Eugenie was a niece of King Edward VII of Britain.	Forged an Anglo-Spanish defensive alliance; introduced the hemophilia gene into the Spanish Bourbon line through Queen Victoria's lineage.