Answer:
Schoenberg's shift from writing a massive opera to creating 12-tone serialism was influenced by his growing dissatisfaction with traditional tonality, his work on the unfinished opera "Moses und Aron," the impact of World War I, and his development of atonality around 1908. These factors collectively led him to seek a new method of composition, resulting in the creation of 12-tone serialism in the early 1920s.
Explanation: