As early as 1935, producer Hal B. Wallis had proposed a film about Maximilian and Juárez to director Max Reinhardt. At the time, he was interested in casting Luther Adler as the Mexican president. In 1937, Wallis and Jack L. Warner, in an effort to dissuade any other studios from embarking upon a similar project, purchased the screen rights to both the novel "The Phantom Crown" by Bertita Harding and the play "Juarez and Maximilian" by Franz Werfel, and on September 30 Aeneas MacKenzie began writing a first draft under associate producer Henry Blanke's supervision. According to Blanke, "Our problem from the outset in preparing this story for the screen was by no means one of glossing over facts, but rather one of cleaving to the exact line." To ensure the film was as accurate as possible, Warners head of research Herman Lissauer acquired three hundred books on the subject, and two historians were hired to help with changes to the script. Because Wallis had decided to cast Paul Muni, then one of the studio's most prestigious contract players, as Juárez, MacKenzie was instructed to make the role the most dominant in the film. His initial script was long enough for two films, and John Huston and Wolfgang Reinhardt were called in to help trim it. Abem Finkel, who had contributed to the screenplays for "Marked Woman" and "Jezebel", worked on the dialogue but received no screen credit for his efforts.