In the middle ages, Christian-held Spain were split into different kingdoms and realms. Before the eventual unification by the "Catholic Monarchs" (Ferdinand II and Isabela I), other kings have unified multiple kingdoms, but the kingdoms split again between children. For example:
- Ferdinand I unified Leon and Castile but "by his will, [he] divided his kingdom among his three sons: the eldest, Sancho, received Castile; the second, Alfonso, León; and from the latter the region of Galicia was carved off to create a separate state for García." (Wikipedia)
- Alfonso VII inherited the unified kingdoms from his mother, but after his death the throne split between his sons Sancho III of Castile and Ferdinand II of León.
- Alfonso III also unified the kingdom which later split, but at least the reason for the split was explained because his children rebelled, forced him to abdicate and split the kingdom between them.
What was the reason for these recurring split? From geopolitical point of view, this prevented consolidation of the realms in the fight against the Muslims, and caused infightings because the successors were prone to claim each other's thrones.