Not sure of the actors, but I think it starred Paul Newman. He was a prisoner of war with the Germans during WWII, and he was visited by a beautiful German heiress (or someone with an important German title), and a love affair developed. Not sure, but I seem to remember that he was her guest at her estate. Might have the details a little off.

The Secret War of Harry Frigg

I knew you folks would come through. Thanks.