The Chocolate Soldier (1941)

Newlywed musical comedy stars Karl Lang (Nelson Eddy) and Maria Lanyi (Rise Stevens) can't duplicate their success on stage once they're married, due to Karl's doubts over Maria's devotion. Convinced that Maria cares more for romance than him, Karl decides to test her by disguising himself as a Russian soldier named Vassily. As Vassily, Karl serenades Maria in public, then flirts with her and sends flowers -- only to grow distressed when it appears that his wife is falling for his alter ego.