Complex sentence: As with beards, sideburns went quickly out of fashion in the early twentieth century, and in the First World War in order to secure a seal on a gas mask, men had to be clean-shaven; this did not affect mustaches.
Split into simpler sentences: As with beards, sideburns went quickly out of fashion in the early twentieth century. In World War I, in order to secure a seal on a gas mask, men had to be clean-shaven; this did not affect mustaches.