Charles V was Holy Roman Emperor (1519-1556), King of Spain and ruler of the Spanish Empire (as Charles I, 1516-1556), Archduke of Austria (as Charles I, 1519-1521), and ruler of the Habsburg Netherlands (1506-1555). The Spanish conquest of the Aztecs and Incas, and the German colonisation of Venezuela both occurred during his reign. Charles V revitalized the medieval concept of the universal monarchy of Charlemagne and travelled from city to city, with no single fixed capital: overall he spent 28 years in the Habsburg Netherlands (primarily Bruxelles), 18 years in Spain (notably Toledo and Extremadura) and 9 years in Germany. After four decades of incessant warfare with the Kingdom of France, the Ottoman Empire, and the Protestants, Charles V abandoned his multi-national project with a series of abdications between 1554 and 1556 in favor of his son Philip II of Spain and brother Ferdinand I of Austria. The personal union of his European and American territories, spanning over nearly 4 million square kilometres, was the first collection of realms to be defined as "the empire on which the sun never sets".