In 1861, Matthew Brady, a well-known portrait photographer, approached President Lincoln requesting permission to move freely about the country photographing the Civil War. Lincoln granted him permission to travel anywhere with the Union armies, and his record of this conflict brought home to millions the horrors of war.
Brady wasn't the first official war photographer. Six years earlier, Roger Fenton, a lawyer and amateur photographer, had returned from the Crimea, having been personally chosen by Queen Victoria and Prince Albert. However, his instructions were more likely to have been to send back work that boosted morale back home rather than the terrible realities of war.
Brady's coverage of the war made him a household name, but he had hundreds of assistants, and it's even possible that he didn't take any of the 7,000 pictures that were marketed under his name. But no one else could have organized the large army of photographers needed to cover the broad sweep of the war and provided access to many leading generals and politicians.