The Dark Angel.

Kitty Vane, Alan Trent, and Gerald Shannon have been inseparable friends since childhood. Kitty has always known she would marry one of them, but has waited until the beginning of World War I before finally choosing Alan. Gerald graciously gives them his blessing. Then, Gerald and Alan go to war. Angered over a misunderstanding involving Alan and Kitty, Gerald sends Alan on a dangerous mission that will change all their lives forever.