Original: The two young monarchs were initially obliged to fight a civil war against Joan , princess of Castile ( also known as Juana la Beltraneja ) , the purported daughter of Henry IV , and were swiftly successful .
Simplified: For the first time since the 8th century this created a single political unit which might be called Spain , although the various territories were not properly administered as a single unit until the 18th century .