The "purple Host" Emily Dickinson refers to in this poem are the soldiers of the victorious army that "took the Flag today," that is, won the most recent battle. Although they are outwardly successful, they nevertheless don't truly understand the meaning of success. The ones who really understand success, the poem insists, are those who didn't experience it. To those who lie on the battlefield "defeated—dying," the meaning of success is infinitely clearer than to those who are off noisily celebrating their triumph. This seems paradoxical, for the victor should be the one to understand success, while the loser, who hasn't experienced it, should not be able to truly "comprehend" it until he tastes it for himself.
The point the poem makes is that the longing of not achieving a goal one has striven for allows one to understand just how sweet success is. Dickinson probably didn't have soldiers and battles in mind as she wrote the poem—they serve only as the analogy. The success she was really thinking of might have been success in publishing her poetry or success in romantic relationships. A person who is repeatedly passed over in any endeavor becomes more and more fixated on achievement and imagines how sweet ultimately achieving the goal must be. This savoring of the "nectar" in one's anticipations is required in order to fully "comprehend" it, that is, to appreciate and know it deeply.
Dickinson's perspective in this poem is consistent with an intuitive personality. According to Myers and Briggs, those with "sensing" personalities trust what they experience with their senses, while those who have greater "intuition" depend more on impressions and metaphors than on their experiences. In this poem, Dickinson describes an intuitive interpretation of success rather than one based on sensory experience alone.
This excellent poem by Emily Dickinson explores and explains the ironical nature of comprehending success. According to Dickinson, success is only something that can be fully savoured or understood by, ironically, someone who does not succeed. Note the way the second and third stanza gives an example to support this argument by referring to a victorious army, the "purple Host," who wins a battle, yet cannot understand and comprehend success as much as a "defeated" and "dying" soldier who lies on the ground, listening to the victory celebrations of his enemies:
Not one of all the purple Host
Who took the flag today
Can tell the definition
So clear of Victory
Thus, Dickinson argues, a victorious army is not able to "tell the definition" of their "Victory" or understand the nature of their success compared to their defeated enemies. To really understand success, Dickinson seems to argue, you have to paradoxically not attain it.