America's first anti-sex trafficking law, the 1910 Mann Act, made it illegal to transport women over state lines for prostitution "or any other immoral purpose." It was meant to protect women and girls from being seduced or sold into sexual slavery. But, as Jessica Pliley illustrates, its enforcement resulted more often in the policing of women's sexual behavior, reflecting conservative attitudes toward women's roles at home and their movements in public. By citing its mandate to halt illicit sexuality, the fledgling Bureau of Investigation gained entry not only into brothels but also into...