As the name suggests, trench coats are inextricably linked to the military and were an evolutionary sartorial result of the introduction of trench warfare.
Prior to that, British soldiers and officers wore topcoats made of heavy wool. Already a weighty garment when dry, these coats became an extreme liability in the trenches, where they would get soaking wet and caked with mud, which seriously hindered movement and mobility.
Gabardine, a waterproof but breathable cotton fabric invented by Thomas Burberry, became the solution. And what we know as the modern trench coat was born.
Though Burberry’s trench coats were never standard issue in the British Army, they did become highly desirable pieces of optional outerwear among soldiers and officers. They were eventually used by armies around the world and eventually made their way into the general public. (hespokestyle.com)