Steampunk fashion trends tend to reflect a given era in history. Some stick with the London fashion of the Victorian era, and others reach to the railroads that criss-crossed nations all over the world as the most efficient transport of the time. However the Great Wars (World War I and II) changed everything. They changed culture, weapons, technology, borders... and they also created many of the iconic fashion statements that steam lovers are intimately familiar with.
Perhaps the most famous garment comes from the first World War, circa 1901 in England, and that is the infamous trenchcoat. You see, up until that war, officers wore greatcoats made of serge. Burberry, after the creation of gabardine fabric, submitted a design to the British army, and was awarded with the contract. The original trenchcoat had D rings for attaching gear and equipment, and there were also shoulder straps for placing markings of rank. The coats were optional, and could be privately purchased by officers that desired to have one. The coat survived into the Second World War, but shorter field jackets that offered greater maneuverability became the order of the day and the trenchcoat was slowly phased out. Many men, after the war, kept their coats and that was how they became a civilian style as well as a military one.
In addition to war on the ground, the World Wars brought air combat to the forefront of combat. However, while the machines to put pilots in the air were being designed and redesigned, there were no such advancements being made for keeping pilots warm in the frigid temperatures of the skies. As such pilots began looking for coats that would keep them warm and functional while they fought in the clouds, and this lead to a lot of pilots adopting leather jackets with high collars, tight cuffs at the wrists and a belt at the waist to hold in warmth. Leather gloves, flying scarves, caps and goggles were also items that helped provide warmth and keep the wind from distracting a pilot, and as such they've become cemented as the uniform of any daring ace. Even into the second war, when fighters had closed cockpits, many of these items remained because the planes still weren't heated. Over time the heavy coats were also adopted by bomber crews and gunners, hence the name bomber jacket in addition to flight jacket.
Lastly, the Great Wars featured the phasing out of standard cavalry, and the creation of the steel juggernaut of the tank. However, tanks could be just as much a threat to the men in them as to the enemy if they weren't careful. One way to fix this was the invention of tanker boots, which should be a favorite of any steampunk out there. These boots, still issued by the armed forces today, are combat boots that attach with straps rather than with laces. The logic was that the belt straps would hold the boots on tightly, but there would be nothing to get caught in the gears and teeth of a tank's mechanisms, and thus fewer tankers would have their feet chewed off in the course of their duty. Additionally, tanker boots are chemical resistant and come with steel toes, just in case accidents do happen.