When and why did women start shaving their armpits?

I am a movie buff and enjoy watching old movies. Watched a Greta Garbo silent film the other day TCM called the "Temptress.'' It is considered a classic -- like Garbo -- although it seems a bit strange to watch silent films in this day and age. Anyway, she is sleeveless is one scene and had shaved armpits. This was 1926 and got me wondering whether just movie stars shaved in those days or it was the norm. Well, on the Internet, you can find the answer to virtually everything. It turns out that sleeveless dresses came into fashion in 1915 and the fashion magazines promoted the idea of shaving while wearing them. It turns out that women didn't start shaving their legs until the 1940s when movie stars like Betty Grable started showing off their legs.
All this may interest only me but I find it interesting how fashion trends got started. I also find it interesting that in the 1960s when some women started not wearing bras -- they didn't burn them by the way, they threw them in an ashcan -- but they didn't make an issue of having to shave for fashion sake although I think most women like not having hairy armpits and legs even though it may be something of a pain in the neck to actually do the shaving.
Oh, and apparently Muslim women have shaved for hundreds of years -- it was a custom for brides to shave off their body hair for their wedding night -- although since they tend not to show must skin, I don't quite get the point.
Again, it's interesting the info you can find on various fashion issues.
FD