Complex sentence: Once the U.S. entered World War II, comic book sales greatly increased, between 1941 and 1944 comic book sales doubled from 10 million to 20 million copies per month.
Split into simpler sentences: Once the U.S. entered World War II, comic book sales greatly increased. Between 1941 and 1944 comic book sales doubled from 10 million to 20 million copies per month.