What is the main conflict in The Constant Princess by Philippa Gregory?

In 1491, Catalina, who will become known to the world as Katherine, is traveling with her parents on the battlefield. They are fighting for Christianity. Her childhood, spent in conquered lands and watching her parents in one crusade or war after another, will uniquely prepare her on her own quest for the crown of England's queen. From the time she is three years old, she is betrothed to Arthur, Prince of Wales and son of the king of England.