There Will Come Soft Rains : Sara Teasdale.

There will come soft rains and the smell of the ground,
And swallows circling with their shimmering sound;

And frogs in the pools, singing at night,
And wild plum trees in tremulous white,

Robins will wear their feathery fire,
Whistling their whims on a low fence-wire;

And not one will know of the war, not one
Will care at last when it is done.

Not one would mind, neither bird nor tree,
If mankind perished utterly;

And Spring herself, when she woke at dawn,
Would scarcely know that we were gone.

 "There Will Come Soft Rains" is a 12-line poem by Sara Teasdale in her collection Flame and Shadow, published in 1920 (see 1920 in poetry). The poem imagines nature reclaiming a battlefield after the fighting is finished. The poem also alludes to the idea of human extinction by war (lines 10 and 12), which was not a commonplace idea until the invention of nuclear weapons, 25 years later. 
Thanks to Wikipedia.