Swedish actor Nils Asther played Gen. Yen. Asian actors were never cast in lead roles in American productions at the time. Although multilingual, Asther did not speak Mandarin. However, he did use a Mandarin dialect for the part.
Chinese embassy officials in Washington, DC, complained about the depiction of the treatment of war prisoners in this film (although many real Chinese warlords were known to torture and execute captured enemy soldiers, this treatment was toned down a bit in the film) and some dehumanizing language about the Chinese people, such as "Human life is the cheapest thing in China," (which remains in the film).
Grace Zaring Stone--who wrote the novel the film was based on--and her husband and daughter were invited to visit the set on one day of filming. She later said she was impressed by the realism of the set but that the film was miscast and Barbara Stanwyck was all wrong for the part of Megan.
Herbert Brenon was originally hired to direct, but he was let go after missing meetings and butting heads with Columbia Pictures chief Harry Cohn. Frank Capra jumped on the project right away. Once Capra was assigned to direct, he made sure that Barbara Stanwyck got the role of Megan.
The trivia item below may give away important plot points.
In the original script Gen. Yen decided at the last minute not to go through with his suicide. The finished film's ending, in which he does commit suicide, seems to have been written after Barbara Stanwyck was cast as Megan.