Excerpts from Books and Wikipedia
"Shortly after the death of his grandfather Ferdinand II on January 23, 1516, Charles succeeded to the Spanish throne. He had an impressive pedigree, and three years later he became Holy Roman Emperor. Charles, duke of Burgundy, was grandson of the Holy Roman Emperor Maximilian I and of the 'Catholic kings' Ferdinand and Isabella I. . . . In 1504 the death of Isabella stripped Ferdinand of his joint ruler's title and Castile passed to Juana, their daughter. She was married to Archduke Philip of Burgundy, thus connecting the fate of Spain to the court of Burgundy. Courtly intrigue over rights of succession ensured negotiations between Philip and his father-in-law were tense. However, Ferdinand stepped down in favor of his 'most beloved children.' This might have been the end of it had Philip not suddenly died, leaving Juana as sole heir to the Spanish throne. Concerns over her mental health ensured her eldest son, Charles, was entitled to act as regent . . . In 1518 Charles I became Holy Roman Emperor Charles V. His realms stretched from the Philippines to Peru and much of central and western Europe, but the troubled succession had only just started." [1001 Days]