Gender Neutral Fashion...Who Started The Trend?

In the history of the Western world women had always worn skirts/dresses and men have always worn trousers and that got me to thinking…When did the whole gender neutral fashion trend begin & who started it?I know that it wasn’t until the time of World War II ‘1939 to 1945’ that women even fathomed the idea of wearing pants and actually began borrowing their husbands when they had to take on