In 1580, Ranuccio I was first in the succession line to the Kingdom of Portugal. His great-uncle Henry I of Portugal's death triggered the struggle for the throne of Portugal when Ranuccio was 11 years old. As the son of the late elder daughter of Infante Edward, 4th Duke of Guimarães, the only son of King Manuel I whose legitimate descendants survived at that time, Ranuccio was according to the succession law the first heir to the throne of Portugal. However, his father was an ally and even a subject of the Spanish king, another contender, so Ranuccio's rights were not very forcibly claimed.