The Battle Hymn of the Republic by Julia Ward Howe

Julia Ward Howe's (1819-1910) poem has a particularly interesting story, in that she wrote it as an alternate set of lyrics to a fairly vulgar Union song during the Civil War.  In doing so, she created a piece of poetry that transcended the form to become an important piece of Americana.  

For more on Julia Ward Howe, please visit her biography at the Poetry Foundation website.