The Battle Hymn of the Republic (arr. Ken Burns)

This stirring anthem was written during the American Civil War by Julia Ward Howe, a leader in women's rights and an ardent foe of slavery. Julia toured a Union Army Camp in Virginia and heard soldiers singing. As Robert J. Morgan relates in Then Sings My Soul, "The music was rousing, but the words needed improvement. Julia's pastor, who accompanied her, asked her to consider writing new and better verses. That night, after the Howes retired to their room at the Willard Hotel, the words came.
'I went to bed and slept as usual, but awoke the next morning in the in the gray of the early dawn, and to my astonishment found that the wished-for lines were arranging themselves in my brain. I lay quite still until the last verse had completed itself in my thoughts, then hastily arose, saying to myself I shall lose this if I don't write it down immediately. I searched for an old sheet of paper and an old stub of a pen which I had had the night before and began to scrawl the lines almost without looking as I learned to do by often scratching down verses in the darkened room when my little children were sleeping. Having completed this, I lay down again and fell asleep, but not before feeling that something of importance had happened to me.'"
Considering how many people have been inspired by Julia's words in the years since, her feeling that "something of importance had happened" was spot on.