The inspiration for this painting is a 14th Century poem called "Triumphs" by Francis Petrarch. Here are a few of my favorite lines:
How near you are your end; behold, I am...
Millions of dead heap'd on th' adjacent plain;
No verse nor prose may comprehend the slain
As in those trifling follies not to trust;
And if they be deceived, in end 'tis just:
Ah! more than blind, what gain you by your toil?
You must return once to your mother's soil
How many moaning plaints, what store of cries
Were utter'd there, when Fate shut those fair eyes
For which so oft I sung; whose beauty burn'd
My tortured heart so long; while others mourn'd
By force extinguish'd, but as lights decay,
And undiscerned waste themselves away
If you would like to read the poem in full, check out "The Sonnets, Triumphs, and Other Poems of Petrarch" for free via Project Gutenberg. That is where I got these lines from.