Charles I of Spain was also Charles V of the Holy Roman Empire. He was known as the “Emperor” or the “Caesar”. As Charles I, he first reigned together with his mother, Joanna I of Castille, until 1555, and was king of all the Hispanic reigns and territories, reuniting for the first time in the same person the crowns of Castille (including the kingdom of Navarre) and Aragon. He became Holy Roman Emperor under the name of Charles V on June 28, 1519.
As a child, Charles received a humanist and Christian education from the Flemish court. He received through diverse inheritances a huge compound of territories and acquired the imperial election in 1519 (disputed by Francis I of France). Charles set out, with the help of his counsellors, how he could use this immense power. His major concerns became: providentialism and the national interest; universalism and the different places that could be considered as the center of his Empire (Flanders, Burgundy, Germany, Castile…); the expansion of his empire towards the Americas, North of Africa, Italy and Eastern Europe; his allies (Portugal and England) and his enemies (Protestants, Turks, French, and even the Pope).
After the death of his grandfather, Maximilian I of Habsburg, emperor of the Holy Roman Empire, on January 12, 1519, Charles immediately inherited the four territories that came from his four grandparents. These were Habsburg (from Maximilian I), Burgundy (from Mary of Burgundy), Aragon (from Ferdinand the Catholic) and Castile (from Isabella the Catholic). A few years later he would renounce his territories in Austria and leave them to his brother.
Emperor of the Roman Empire (Imperator Romanorum) was a title given to an elected and crowned monarch of the Holy Roman German Empire. Its first emperor was Charlemagne, crowned in Rome on December 25, 800. The last emperor was Francis II, who abdicated under the pressure of the French emperor Napoleon I on August 6, 1806.
The aspirant to the imperial crown was chosen by the German princes and named King of the Romans, while the actual emperor lived. Once crowned as King of Romans in Germany, the aspirant had to travel to Rome in order to be crowned by the pope. Due to the internal difficulties that took place in the Empire, the travel could take years and there were kings that died before getting to crowned in Rome. The Emperors of the Holy Roman Empire where crowned by the pope until the 16th century. From then on, as soon as the predecessor died, the successor was automatically elected emperor.
As for Charles I of Spain, on June 28, 1519, he was crowned King of Romans, title that made him de facto Emperor of the Holy Roman Empire, although he was not crowned by Pope Clement VII until February 24, 1530 in Boulogne.