It is a traditional song from England. It is older than the civil war of the Americas. They get confused in the United States and borrow from other people's history and unknowingly think it is theirs.

The cruel war is raging, Johnny has to fight
I long to be with him from morning 'till night
I want to be with him, it grieves my heart so
Won't you let me come with you? No, my love, no.

Tomorrow is Sunday, Monday is the day
That your captain will call you and you must obey
Your captain will call you, it grieves my heart so
Won't you let me come with you? No, my love, no.

I'll tie back my hair, men's clothing I'll put on
I'll pass for your comrade as we march along
I'll pass for your comrade, no one will ever know
Won't you let me come with you? No, my love, no.

Oh Johnny, oh Johnny, I feel you are unkind
I love you far better than all of mankind
I love you far better than words can e'er express
Won't you let me come with you? Yes, my love, yes.

They marched into battle, she never left his side
'Til a bullet shell struck her and love was denied
A bullet shell struck her, tears came to Johnny's eyes
As he knelt down beside her, she silently died.