What is the main conflict in Marie Antoinette: The Journey by Antonia Fraser?

Antoinette is the fifteenth child of the Austrian King and Queen. Her childhood is filled with instruction based on the probability that she is destined for a royal marriage to cement an alliance between countries and families. This happens to be the case when her older sister, pledged in marriage to the Dauphin of France, dies and leaves Antoinette to fill that role.