World War I was the first armed conflict documented through photographs. At the beginning of the twentieth century, the first “mass-market” cameras were created, but the 35mm film camera prototype, initially known as “Ur-Leica”, was developed in the decades of 1910-1920. The War was an ideal event to use this new technology, as a fast way of documenting events.
For more details about the history of photography during this period:
Carmichael, J. Photographers of the First World War (London: Routledge, 1989)
There are several websites that have interesting photo collections, here are some suggestions:
The Atlantic: http://www.theatlantic.com/static/infocus/wwi/
National Geographic: http://ngm.nationalgeographic.com/2014/08/ww1-underground/gusky-photography
Great War (independent site): http://www.gwpda.org/photos/