Success is counted sweetest
By those who ne'er succeed
To comprehend a nectar
Requires sorest need
Not one of all the purple Host
Who took the Flag to-day
Can tell the definition
So clear, of Victory
As he, defeated, dying
On whose forbidden ear
The distant strains of triumph
The “purple Host” is a  metaphor  for royalty and, in this context, the capitalised “Victory”. They have taken the Flag — an example of  metonymy  — which is representative of victory on a battle field, but also representative of success in life. 
 Note the word “Host” is archaic, meaning army or military force.  The capital “H” and the archaism add to the significance. It represents every conquering army across the centuries, as well as every successful human endeavour.
Success is counted Sweetest