What was the role of photography in making the Civil War the first "modern war"?What were other factors that made the Civil War a modern war?
Indeed, in many ways, the US Civil War of 1861 was probably one of the earliest 'modern' wars.
One major reason for this was the major use of photography and photographic journalism for the first time, properly, in world history, so, along with the general media expansion at this time, updated accounts of the war/fighting reached people very quickly indeed, throughout the nation. This rapid proliferation of news, especially war/conflict, is a prominent feature of modern time warfare and 'war journalism'. I have given a basic link below, which might be useful in relating photography --especially war photography-- to media and journalism/reportage.
Other significant factors that reflected the 'modern' nature of this war included (a) technological developments, especially in arms/weapons (b) large-scale financial cost/investment in military (c) proper, organised propaganda campaigns by both sides, to encourage/promote their own points of view and demoralize opposition and so on. Im certain that if you make a deeper and more detailed study of the topic via library research, and your own textual readings, you can identify other similar factors too, which are common to latter-day wars but werent there in old style warfare.