I and My True Love — Helen MacInnes

Payton and Sylvia Pleydell were the most respected, brilliant couple in Washington. Theirs seemed a perfect marriage. But Kate Jerold, newly arrived from California, soon saw different. Why was her cousin Sylvia so secretive? And was her husband really as devoted as he seemed?
Kate's visit coincided with an unexpected arrival. The Czech, Jan Brovic, was with them during the War, and he and Sylvia had been deeply, secretly in love. Now, six years later, there was a new kind of war and former allies had become bitter enemies. Now Jan was on the other side and it was no longer safe to be his friend. But, after six years, they were still terribly, hopelessly in love....