I remember seeing somewhere that Hitler was caught under a surprise gas attack in WW1, but was unprepared due to not being able to put on his mask on properly becuase of his long mustache. He then shaved it shorter for the mask to fit properly. Is this the real reason why he started shaving his mustache short, or was it just a personal choice?
It may be true that gas and his experience as a soldier in WWI had something to do with his moustache. In order for a gas mask to be effective its important that a tight seal is made around the wearer's mouth. During WWI the most popular facial hair trend was styled after Kaiser Wilhelm, it was a handlebar-type moustache but with a little extra fluff around the edges. I know this is a terrible description but have a look on Google images and you'll see! The hair around the edges of the mask prevented the mask from sealing around the face and therefore would allow gas to be inhaled. If Hitler was sporting this style its possible that he may have had a close call with his mask not attaching, or had seen other soldiers suffer because of the same malfunction.
I googled the question and found this link: https://www.quora.com/Why-did-Hitler-make-his-moustache-like-that-Did-he-copy-from-Charlie-Chaplin (he certainly didn't copy Charlie Chaplin) and it provides an interesting perspective about the 'Toothbrush' moustache that Hitler sported, positing that it was an easier to maintain version of Kaiser Wilhelm's. I hope that helps! I'm a history teacher and I feel kind of embarrassed now that I've never tried to understand anything about one of the most iconic moustaches in history! Happy New Year :)