As I ponder'd in silence,
Returning upon my poems, considering, lingering long,
A Phantom arose before me with distrustful aspect,
Terrible in beauty, age, and power,
The genius of poets of old lands,
As to me directing like flame its eyes,
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
The ghost is asking him if he is prepared to be a great poet. He knows that to be great he has to sing about great War (that is, with the capital W, universalized) and brave soldiers. The example came from the great old poems that Whitman read during his life like Virgil’s  Aeneid  and Homer’s  Odyssey . War is the most noble theme in poetic tradition. 
 The ghost, which also represents Whitman and his poet’s spirit, is challenging Walt. Is he capable of doing it? Is he ready to fight against himself and the contemporary “intellighenzia”? 
 The last line is about the birth of a perfect soldier that is simultaneously himself and the humankind he’s singing about.
As I Ponderd in Silence