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: The two young monarchs had initially to fight a civil war against Juana , princess of Castile ( also known as Juana la Beltraneja ) , the purported daughter of Henry IV .