in short The Great Depression, American Cinema, return of the waist, long backless dresses, Marlene Dietrich & trousers
After the Wall Street crash in 1929 also fashion went through a sudden change. During the years of depression, fashion was driven by the fantasies of Hollywood and by the desire to return to a more traditionally feminine image of women.
The boyish look that has dominated the 20s disappeared completely and women were now women again.
World War II changed every aspect of civilian life. Women had to replace their husbands and work for themselves. Clothing was no longer a personal concern and what you wore, the textiles and costs were controlled.
A different icon of that era is Rosie the Riveter, representing US women working in factories during WWII. You might have come across this poster before as it is commonly used as a symbol for feminism.
But also the blue romper and the bandana is often copied!