Spain becomes a united kingdom under the 15 year old Charles I.
Charles was the heir of three of Europe’s leading dynasties: Valois of Burgundy, Habsburg of Austria, and Trastámara of Spain. As heir of the House of Burgundy, he inherited areas in the Netherlands and around the eastern border of France. As the head of the House of Habsburg, he inherited Austria and other lands in central Europe, and was also elected to succeed his grandfather, Maximilian I, as Holy Roman Emperor. As a grandson of the Catholic Monarchs of Spain, from the Spanish House of Trastámara he inherited the Crown of Castile, which was developing a nascent empire in the Americas and Asia, and the Crown of Aragon, which included a Mediterranean empire extending to southern Italy.
Charles was the first king to rule Castile and Aragon simultaneously in his own right (as a unified Spain), and as a result he is often referred to as the first king of Spain. The personal union under Charles of the Holy Roman Empire with the Spanish Empire was the closest Europe has come to a universal monarchy since the time of Charlemagne in the 9th century.
Image: Charles V in 1516