Louis Untermeyer - Reveille

What sudden bugle calls us in the night
   And wakes us from a dream that we had shaped;
Flinging us sharply up against a fight
   We thought we had escaped.

It is no easy waking, and we win
   No final peace; our victories are few.
But still imperative forces pull us in
   And sweep us somehow through.

Summoned by a supreme and confident power
   That wakes our sleeping courage like a blow,
We rise, half-shaken, to the challenging hour,
   And answer it -- and go.

Louis Untermeyer

http://www.poemhunter.com/poem/reveille-3/