Final answer:
Movies were considered part of a decisive turn toward the "modern" in American cultural values in the late 19th and early 20th centuries as they created a mass culture and united people across regions. Movies were considered dangerous to some social groups because they challenged traditional values and norms.
Step-by-step explanation:
The development of movie-screen entertainment in the late 19th and early 20th centuries is considered part of a decisive turn toward the "modern" in American cultural values by many historians. This is because movies brought people together from different areas and created a mass culture for the first time, uniting people who were once focused on regional and local concerns.
Movies were considered dangerous to some social groups because they challenged traditional values and norms. The content of movies, especially in the early years, often explored taboo subjects and presented alternative lifestyles. This threatened the established social order and led to concerns about the moral influence of movies on society.