On the surface this is a cheeky letter sent to Sue charging her with not having written in so long that it has hurt her feelings. However, she is also describing how just through the use of words (or lack of them) we can send someone to their death or keep them alive. In a sense she is alluding to John 1:1, “In the beginning was the Word, and the Word was with God, and the Word was God” in that before there was anything else, there was the capacity for language and without language there isn’t life, the universe, or anything. When Emily writes a poem she is, in essence, creating life (creating art), she is giving her emotions and feelings life through art, yet the absence of the words is the death of life and the death of poetry.
Embedded deep within this poem is a pun about keeping one’s word. Emily is frustrated that Sue has not written and has thus kept her word(s) to her self. Usually when we think of the expression keeping our word we think of someone keeping a promise and acting honorably and when someone fails to keep their word they are acting dishonorably; by keeping her word(s) she is not keeping her promise.
To expand on the idea of being honorable, notice how Emily uses the imagery of a soldier, the “epauletted Brother”. When we think of soldiers we think of men (and now women) who swear an oath to uphold a certain set of values and will do everything in their power to prevent others from disrupting those values. However, it is really words that make up the fabric of honor: an oath is a collection of words, and when a soldier is in battle they will put their lives at great risk all because they were ordered to (through the words of a superior). Thus when a soldier is killed on the battlefield, they are not only killed by the “sword” but also by the “barbed syllables” of their captain’s orders and, more generally, by the propaganda of their nation (and because of the propaganda of the opposing side).
Yet the soldier who dies for the losing side is the soldier who is “forgot”. Their side’s words no longer “Can pierce an armed man” because “Time” has forgotten where the words (and the soldiers) fell on the battlefield. All that remains is a ghostly image, what Emily refers to as “Time’s sublimest target” which can mean to rise “to a great height” (OED) but also can mean to be transformed into a “vapor” (OED). Thus the soldier who dies in defeat is “Time’s sublimest target” in that they are relegated to little more than a ghost memory, while the soldier who is victorious is lifted up and praised. Time is kind to one and forgets the other.
Thus when we keep our words to ourselves, we fail to give life to each other, we cannot inspire or order or declare our intentions, we are basically ghosts, but when we give our words we breathe life into each other, even those who may have died a long time ago such as a soldier who once fell in battle but is now remembered through a poem. The “word” is life and to withhold the word is to withhold the life.