Before World War I, fashion's use of military elements was mostly decorative: buttons and brocade; intricate mohair braiding; short, stiff collars. Beginning with World War I, however, the relationship started to take practical form. The British boys in the trenches needed warm, heavy, water-resistant coats, and Thomas Burberry offered to make them. (Actually, the Aquascutum fashion house may have designed the trench coat first, but that's a whole different battle.)
The fashion industry, like every other one, was affected by and involved in the war effort, and the famous Burberry trench coat was on the battlefield before it ever graced Fifth Avenue.
The jump to civilian fashion came later, when the war ended, and it turned out Burberry's gabardine coat, with all its military bells and whistles, did well in peace time, too. By the mid-1940s, the trench coat was utterly stylish.
Both of the World Wars were fashion fodder when it came to outerwear. Bomber jackets, field jackets and pea coats all started as military issue. Fashion designers, attracted to the military-grade function, adopted the forms and adapted them to suit civilian style -- adaptation that grew increasingly artful as peacetime set in, fabric supplies were renewed, and designers returned full-force to their stylish day jobs ...