A class of chemicals known as sulphomanides were the first antibiotics to be discovered and used in a systematic way. They were derived from a drug called Protosil, discovered by the Bayer Corporation in 1932. This was actually a red dye, which had no effect on bacteria outside of a living organism. Inside one, the dye was broken down into several molecules, one of which had the antibiotic effect. This molecule was isolated, and called Sulphanilimide. Powdered Sulphanilimide was used as an antibiotic during WW2, especially in combat situations, as you mention. It's use greatly reduced the risk of infection from wounds. As it's systematic name is too complicated for daily use, it was commonly shortened to Sulfa. Nowadays, penicillin derivatives and similar chemicals are used for the same purpose, but these were not commonly available at the time. The method for mass-producing penicillin was only discovered in 1941, and even this produced a low yield compared to modern techniques.