4 ideas
The instrument built to control what the soldiers said is the same instrument that preserved it. Any reading of the testimony must therefore treat the watcher's interests as part of the text, not as an obstacle outside it.
Each surviving extract passed through several filters: a mostly non-literate soldier dictated to a scribe, the scribe rendered it in conventional letter-writing forms, a censor translated it into English, and the censor chose which lines to excerpt. Every layer altered or selected the words. What reaches the reader is a composite, and the soldier's own voice has to be inferred through all four hands.
Censors copied passages that bore on morale, loyalty, sedition, and conditions at the front, because that was what their reports to command needed. The corpus is therefore weighted toward what alarmed or reassured the authorities, and the routine domestic content of the letters is underrepresented. Patterns in the collection partly reflect the censor's anxieties rather than the soldiers' priorities.
The soldiers knew their letters would be read, so they self-censored and used metaphor, allegory, and flowery indirection to say what they could not say plainly, such as how badly casualties were running or urging relatives not to enlist. Surveillance does not simply silence people.