ABOVE: “Prisoners from the Front” Winslow Homer (1866)
(“The most telling of all paintings about the Civil War,” according to the New Yorker… >>)

Account given by anonymous captured Confederate officer (“a day or two after Lee’s surrender”, April 1865), speaking to Union officers after listening to Union war songs: 

Gentlemen, if we’d had your songs we’d have licked you out of your boots! Who couldn’t have marched or fought with such songs? … every one of these Yankee songs is full of marching and fighting spirit…

I shall never forget the first time I heard “Rally Round the Flag.”  ‘Twas a nasty night during the ‘Seven Days’ Fight’ [1862], and if I remember right it was raining. I was on picket, when, just before ‘taps,’ some fellow on the [UNION] side struck up that song and others joined in the chorus until it seemed to me the whole Yankee army was singing. Tom B–, who was with me, sung out, ‘Good heavens. Cap, what are those fellows made of, anyway? Here we’ve licked ’em six days running; and now on the eve of the seventh they’re singing “Rally Round the Flag.”

I am not naturally superstitious, but I tell you that song sounded to me like the ‘knell of doom,’ and my heart went down into my boots; and though I’ve tried to do my duty, it has been an up-hill fight with me ever since that night.

Here is the full story, as quoted in Root‘s autobiography:

This mentions a delightful flurry of songs of the partisan variety, by Root & others, which (we can assume) were current at the end of the war:

UNION SONGS:

• Battle Hymn of the Republic
• John Brown’s Body
• We Are Coming Father Abraham
• Tramp Tramp Tramp (The Prisoner’s Hope) [audio >>]
• Star Spangled Banner
• Rally ‘Round the Flag (a.k.a. Battle Cry of Freedom)

SOUTHERN SONGS: 

• Marseillaise
• Bonny Blue Flag
• Dixie
• Maryland, My Maryland [audio >>]
• … Dead March in Saul (?) [Q.V. >>]