Operator 13 (1934)

Gail Loveless, a spy known as Operator 13 working for the Union and Federal cause during the Civil War posing as a Confederate woman named Anne Claybourne, meets and falls in love with Confederate Captain Jack Galliard. Dusguised as an octoroon maid at a Confedarate military ball, she learns and relays secrets to the Union forces that are devastating to the Rebel cause. Assigned to the case to track down and kill the spy maid, Galliard learns she is also the woman he loves, but proceeds with his assignment. He captures her but is in danger of being captured himself as Federal forces are approaching. Gail/Anne saves him and they part to wait until peace comes to resume their romance.