The World's Fair + Olympics = Growth!
The 1904 World Fair was held in Forest Park and spurred much of the development in the CWE as wealthier citizens wanted to move away from downtown. Large single family homes in various styles - followed by apartments and businesses.
Then came the 1904 Olympics - also taking place in Forest Park and on the current campus of Washington University- Francis Field remains as part of the main venue.
Another fun fact = this was the first time English was the official language in the Olympics and the first time they were ever held outside of Europe