It should not be surprising that O'Sullivan put so much thought into his Civil War photographs, as the nature of documenting war and the nature of war itself involve many conceptual problems. The first of which was "What does it mean to document a war?" Matthew Brady said "'I can only describe the destiny that overruled me by saying, like Euphorion, I felt I had to go. A spirit in me said 'go' and I went.'" For Brady, it was a "strange, almost spiritual determination." 9 Yet, most people still think of the Civil War photographers as objectively, if not mindlessly photographing the history in the making all around them. But the project was neither that simple nor that objective.

Given the assignment of recording history, a photographer could either be relieved from having to think very much about his subject or required to think about it all the more. He might simply conclude that anything having to do with the war was a fit subject for photographs documenting it. However, as we have seen, the medium of photography at the time made it impossible to photograph battles, death, or other acts of destruction because of the movement and danger involved. Photographers were thus forced to come up with other ways of portraying the reality of war, and thus to consider the range of forms that war took. The similarity among the Civil War photographs, especially in their subject matter, suggests that the photographers, as a group, quickly came to discover what aspects of the war they could successfully portray given the limits of their medium. In the process, they were forced to think creatively and critically about what war actually was.

Alan Trachtenberg points out that in addition to the limits of their medium, photographers also had to take into account a precedent of image making set by painters.10 Painters had portrayed war by portraying the glory of its heroes and the drama of its battles. Photography could neither capture the heat of battle, nor the moment of a hero's victory. It could not even idealize heroes in the way that audiences of paintings would have been used to seeing them. Instead, "We see the war not as heroic action in a grand style but as rotting corpses, shattered trees and rocks, weary soldiers in mud-covered uniforms or lying wounded in field hospitals - as boredom and pain."11 The limitations of subject were transformed into a style of realism, portraying of the reality of war as mundane and quotidian, and as an event in real space and time.12 This was not only a new style of realism, but a new definition of the reality of war.

Central to the photographer's project was a need to interrogate the meaning and reality of war, the need to investigate methods of representing this reality, and a recognition of the ways in which representative modes defined the meaning and reality of their subject.

In addition to limited subject matter and style, the limits of the picture's frame created another obstacle to portraying the reality of war. With 10,455 military actions occurring in four years, in eighteen states and territories at the rate of 6.5 actions per day, 13photographers could only hope to portray a fraction of the reality of the Civil War. Photographers often made their pictures to be used in periodicals or collections that would provide literary contexts for viewing and understanding them. Furthermore, captions could also tell a viewer how to read a photograph. However, it is likely that the photographer surrendered the authorship of such a caption to the newspaper or collector such as Mathew Brady to whom he sold his photograph. Whatever the use of the photographs, the project of documentation provided photographers with the challenge of making one image stand on its own as a representation of the war as much as possible.

Photographers like O'Sullivan were limited in their ability to portray the totality of the war in not only its physical manifestation, but in its conceptual and emotional significance as well. Trachtenberg writes:

The reality they depict is the reality of violence...But do they show 'war'?...Can any single photographic image bring home the reality of the whole event? The same question can be asked of photographs of any event, it might be said. Yes, but in the case of war the normal gap between sense experience and mental comprehension is stretched to an extreme.14

Trachtenberg suggests that because the Civil War photographers' subject was so difficult to comprehend, it brought to the forefront the limitations of the medium to truly represent the reality of its subject accurately or fully. But Trachtenberg also suggests that in addition to the medium, the complexity of war contributed to the limited success with which it could be portrayed:

A more general problem posed by wars waged by nation-states in the nineteenth century [was]...How can one experience the chaos of war and describe it rationally at the same time? How was it possible to see photographically, in single, segmented images, and to see politically or historically, as it were, with an eye to the meaning of the transcribed scenes, their meaning within a war itself so difficult to see intelligibly?15

Photographers including O'Sullivan were faced with the difficulty of portraying photographically something that was much more than just visual, but political and historical as well. According to both Trachtenberg and Timothy Sweet, they failed in many respects. They argue that the photographs' fragmentary nature, idealization of the union, and naturalization of death caused a "loss of clarity about both the overall form of battles and the unfolding war as such and the political meaning of events."16 At the same time, Trachtenberg concludes that fragmented representations of the war also suggested the inexpressibility of war which was as true an aspect of war as its physical or ideological battles.17 The challenge of representing the war in photograph not only initiated the photographer into the questions of reality and representation, but suggested that the reality of things is so complex as to defy a single representation.