Airplanes first appeared in World War I, but they played a much more significant role in the combat of World War II. They were initially used for reconnaissance in World War I, but in World War II, they were used for bombing and air-to-air combat. Poison gas and Gatling guns were also used in World War I, but their use was much less prevalent in World War II. Camouflage uniforms were used in both World War I and World War II, but they did not play as significant a role in combat as airplanes.
Answer: Airplanes.