After researching many scholars' opinions on the notion of the "purple Host" in Emily Dickinson's poem, the truth of the matter is that we can only speculate the true meaning of its symbolism. However, because of the time period of the poem and the surrounding context, we can have a pretty good idea.
Here is the stanza of text that reveals the image:
Not one of all the purple Host
Who took the Flag today
Can tell the definition
So clear of Victory
Emily Dickinson spends the entire poem exploring ideas through their opposites. In this case, she is talking about the idea of "victory" specifically in battle, which it can be assumed is in reference to the victory of the Northern Army in the Civil War. The image of the "purple Host" has two words within it, so let's take each in turn.
First, the word "purple." Throughout history (especially early history), the word purple was meant to symbolize royalty. Long ago, in England, it was the most coveted cloth because the dye could only be made by grinding tiny sea snails. So we can say that Emily Dickinson might want us to think about the "royal" nature of those who were victorious, the North, "who took the flag today." However, purple can also be used to represent the bloodshed in battle. Therefore, because the "Host" is "purple," perhaps it was a battle that wasn't easily won. Nevertheless, the "purple Host" will still have not as good of an understanding of victory as the losing side. So, perhaps, we can also say that Emily Dickinson might want us to think about the "bloody" nature of those who were victorious, the North, "who took the flag today" ... and not easily.
Second, the word "Host." Here we need to explore three different meanings behind this metaphor for the Northern Army: the Host is in charge, the Host has won the battle, and the Host can even be considered holy. A "host" at a party is the person who has invited the attendees. If you take this particular symbolism, then it was a bloody battle that the North won on their own turf. Possible. Now let's take the idea that the word "Host" is actually capitalized. This could further prove that the North was truly victorious in battle, giving them almost a proper noun feel to the word "host." They have taken "the flag today," after all, ... and won. On the more ethereal side, however, we have to understand the Christian idea behind the word "Host." In Roman Catholicism, the "Host" is the bread of Communion that has truly been turned into the flesh of Jesus Christ for us to eat. Even though the word "Host" does not at all mean God here, it could mean that the North can be considered just as "holy" with God on their side through their victory.
The irony of all of this is, even through all of this grand symbolism of the North ... from being "royal" to "holy," those soldiers still don't understand the concept of victory as much as the South. Why? The answer is found here:
As he defeated--dying--
On whose forbidden ear
The distant strains of triumph
Burst agonized and clear!