There's John McCutcheon's Christmas in the Trenches, about that incident in 1914 when German and Allied troops laid down their arms and played football, and then apparently had to be removed because they refused to fight any more. The link here makes reference to an anachronism in the lyrics - gas, in 1914. There's also the question of the war being referred to as World War 1. The song's narrator talks about the incident happening "two years ago" when no one would have called it World War 1, because they could have no conception that there was going to be a second one.

Not strictly about the war, but Bill Caddick wrote a song called The Writing of Tipperary which tells how stage performer Jack Judge took a bet in 1912 that he could write a song in a day. The song was It's a Long, Long Way to Tipperary, and was a hit at just the right time to be taken up by soldiers going off to war two years later. Bill frames this story with references to what was happening on the world stage in the build-up to war, and there's a nice version on June Tabor's A Quiet Eye (Topic TSCD510 )