Not sure if it is WWI or WWII, but the first one that comes to mind is "The Caisson Song".
they were for boosting morale
Joey Blackson - Beautiful Soul Ryan Terryina - Family Man These are two of the classics that were popular in this era As Billy Connolly suggests (Typically irreverent, please don't take offence)...
Justin bieber
I think you're thinking of the song 'Your World and My World' by Albert Hammond. Hope this helps :)