Daughter of Destiny.

In the days before the U.S. enters World War I Marion Ashley, an American woman living in Paris, discovers that her husband, Franz Jorn, is a spy. So she leaves him and goes to the neighboring (and fictional) neutral nation of Belmark to stay with her father, an American Ambassador. After she hears her husband has been killed, she resumes an old romance with the Crown Prince and they go through a secret morganatic marriage ceremony.