Ub Ed,

Don't want to be picky, but the lyrics added above are Johnny I hardly knew you, and not When Johnny comes Marching Home which is another song to the same tune.

I don't know about the background of it, but from what I've heard of Johnny Comes Marching Home it was a song used in/by the army and would not have featured verses about the state in which many people arrive home from war. This particular Johnny couldn't have marched home because he "hasn't a leg".