Original: Married under the joint motto, tanto monta, monta tanto, 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.