But why did Hitler choose this particular style of moustache? An answer may have recently come to light. An unpublished essay, written by another WWI soldier who knew Hitler during this period, wrote that Hitler trimmed his Prussian-style moustache because he was ordered to do so. The wide, somewhat bushy Prussian-style moustache that Hitler had been wearing simply didn't fit under a gas mask. The Telegraph has more about this new moustache discovery.