With finger pointing to many immortal songs,
And menacing voice, What singest thou? it said,
Know'st thou not there is but one theme for ever-enduring bards?
And that is the theme of War, the fortune of battles,
The making of perfect soldiers.
Be it so, then I answer'd,
I too haughty Shade also sing war, and a longer and greater one than any,
Waged in my book with varying fortune, with flight, advance
 and retreat, victory deferr'd and wavering,
(Yet methinks certain, or as good as certain, at the last,) the
field the world,
For life and death, for the Body and for the eternal Soul,
Lo, I too am come, chanting the chant of battles,
I above all promote brave soldiers.
Now the speaker is presenting himself as a poet. He’s ready and determined to give birth to his work because now (after the dialogue with the phantom of poetry) he is ready to compare it with the great classics like  The Iliad  or  The Aeneid . 
 He’s singing about brave soldiers, including humankind in its entirety, and that is the main character of  Leaves of Grass .
As I Ponderd in Silence