 Ultimately, Charles V conceded the Peace of Augsburg and abandoned his multi-national project with a series of abdications in 1556 that divided his hereditary and imperial domains between the Spanish Habsburgs headed by his son Philip II of Spain and the Austrian Habsburgs headed by his brother Ferdinand, who was Archduke of Austria in Charles's name since 1521 and the designated successor as emperor since 1531.The Duchy of Milan and the Habsburg Netherlands were left in personal union to the King of Spain, but remained part of the Holy Roman Empire.The two Habsburg dynasties remained allied until the extinction of the Spanish line.