In this poem the speaker states that success is most valued by those who fail, just as victory in battle seems most precious to a soldier who is defeated and dying. Dickinson uses an analogy to state her claim in the first stanza:
To comprehend a nectar
Requires sorest need.
Here she compares a thirsty person's appreciation of a drink to a loser's appreciation of victory, saying that the thirstier you are the more you are going to value having that thirst quenched. In the same way, those that suffer defeat will crave victory more than the victors.
She goes on to use imagery very effectively in the last stanza to seal her case:
As he defeated - dying -
On whose forbidden ear
The distant strains of triumph
Burst agonised and clear!
Dickinson thus creates an image of a soldier dying on the battlefield (note the alliteration in "defeated - dying" which emphasises the pity of the soldier's state) who is nevertheless able to hear the victory chants and music of the opposition. He has just lost his life in vain and thus, Dickinson argues, is able to comprehend the nature of success far more than the victors.
This three-stanza poem by Emily Dickinson relies on a number of literary devices for its effectiveness. It begins with an aphorism--a statement of truth expressed in a concise, witty manner. "Success is counted sweetest by those who ne'er succeed" makes a good aphorism because it comments philosophically on success, uses only nine words, and uses alliteration and a variation on the word success to please the ear. Additionally, it contains a touch of irony or paradox. Only those who don't succeed fully appreciate success, according to the poet.
The next two lines use metaphor and a version of synesthesia. Success is compared to nectar, but instead of referring to tasting that nectar, Dickinson uses the word comprehend. Synesthesia is type of figurative language in which what can be appreciated through one sense is described with another, such as describing flowers as melodious. Here, Dickinson uses the brain's ability to understand to mean "appreciate the taste of."
The next two stanzas use analogy to illustrate the aphorism. An analogy is an extended metaphor. Dickinson uses a loss on the battlefield to show that the defeated party appreciates victory in a way that the victor cannot.
Stanza two also makes use of metonymy, which uses something associated with another thing to represent the thing. Here "took the Flag" is a figurative way of saying "won the battle." Obviously merely taking a flag cannot constitute a successful outcome in a war, but the figure of speech is used here to indicate the triumph of the winning side.
Finally, Dickinson uses sound devices in this poem. She uses predominantly iambic trimeter rhythm and meter. Every other line rhymes. Alliteration occurs in lines 1 and 2 with the repeated /s/ sound; lines 3 and 4 with the repeated /n/ sound; line 6 with the repeated /t/ sound; and line 9 with the repeated /d/ sound. Assonance is evident in the first stanza with repeated long /e/ sounds and in the final stanza with repeated long /i/ sounds.
Dickinson skillfully uses figurative language and sound devices to convey her ideas in this brilliant little poem.