Photography has been used to document wars since the American-Mexican War of 1846-48, when daguerreotypes were used.
Of course, it was the Crimean War of the 1850s that was the first war substantially documenting the ravages of conflict.
The American Civil War was also highly documented, and it and the Chinese Opium War of 1860 were the first conflicts to actually show dead bodies.
Of course, other later conflicts were recorded by photographers, including the Italian Papel Wars, the Franco-Prussian War, the Romanian War of Independence, the Russo-Japanese War, WWI and WWII, the Spanish civil war, plus the Korean and Vietnamese conflicts.
War photography has been used to proselytize for and against, as propaganda, and as documentation. But the horrors of conflict seem to always come through.