Photojournalism was sparked into existence the instance photography itself was invented. In fact, artists had already been using the Camera Obscura for centuries before to paint environments exactly as they appeared. The moment that humans discovered that silver could be used to capture light into an image, and chemicals could be used to fix the image and make it permanent, people began dragging cameras around the world and focusing them on culture. 

The first Documentary Photographers had to carry more than just their cameras into the field, they had to bring actual darkrooms too.  The cumbersome nature of pulling light-tight covered wagons over wild country with teams of horses didn't stop these pioneering photojournalists from documenting all aspects of 19th century life, including war; the American Civil War was widely documented, as was the Crimean War. The advent of dry-plate photography, and then film, eliminated the need to bring a darkroom along, and soon enough everyone would have a camera.