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
Break, agonized and clear!
The second and third stanzas are  enjambed , so the meaning of both are linked, one flowing from the other.  The emphasis is increased by hard, thudding  alliterative  “d"s.  This increases the pace, leading to the emphatic ending of the poem. 
 The stanza describes the defeated in this battle.  The first line depicts a man defeated and dying on a battlefield.  The defeated man’s ear is “forbidden” because as someone who is lost, he does not deserve to hear the sounds of triumph. The sounds, ringing clearly in his ears despite their physical and figurative distance, torment him from where he lies dying, knowing that he did not succeed. 
 Note that the agony the man suffers can apply to the specific circumstances of the battlefield, but also implies wider human suffering; unrequited love, death of a child, other forms of loss. 
 Though Dickinson is able to keep the  metaphor  open to nonliteral interpretations, the dying soldier is a wonderfully suggestive figure in his own right. This is the final time he will watch (or hear) victory fall through his fingers. He dies a failure hearing the victorious songs of the enemy. The third stanza helps to prove and complete the paradox Dickinson has set up in the poem.
Success is counted Sweetest