In his autobiography The Story of a Musical Life, Root relates how “Immediately a song started in my mind” when he heard of President Lincoln’s first call for 300,000 volunteers:
I thought it out that afternoon, and wrote it the next morning at the store. The ink was hardly dry when the Lumbard Brothers — the great singers of the war — came in for something to sing at a war meeting that was to be holden immediately in the court-house square just opposite. They went through the new song once, and then hastened to the steps of the court-house, followed by a crowd that had gathered while the practice was going on. Then Jule’s magnificent voice gave out the song, and Frank’s trumpet tones led the refrain —
‘The Union forever, hurrah, boys, hurrah!’
and at the fourth verse a thousand voices were joining in the chorus. …” — p. 132-133
This anecdote gives us a sense of the breathless pace of events in the first year of the war, even far from the front lines.
Here’s a colored print which, strangely enough, lists a (handwritten) 1861 publication date (?):
By 1864, the song is being used as a rallying cry in the presidential campaign, which had become a referendum on Lincoln’s handling of the war and the Union cause itself:
“We are ready for the polls…”