The reign of John III was marked by active diplomacy. With Spain, he made alliances through marriage that ensured peace in the Iberian Peninsula for a number of years. He himself married Catherine of Austria, the daughter of Philip I of Castile. His sister Isabella of Portugal married Charles V, the king of Spain and Holy Roman Emperor. His daughter Maria Manuela married King Philip II of Spain - and there were others. However, the intermarriage of these closely related royal families may have been one of the factors that contributed to the poor health of John's children and of future King Sebastian of Portugal.