“The soldiers briefly exchanged some words and then they too started singing vociferously.”
The soldiers in the tavern in Steinburg in the story Fog of War only sang the first verse:
Es waren drei Soldaten
es war ein junges Blut
sie hätten sich Eins begangen
der Marschall hätt sie gefangen
gefangen mußten sie sein
Translation:
There were once three soldiers
One was a young blood
They broke a rule
The marshal took them prisoner
They had to be taken prisoner
This version of Es waren drei Soldaten (There were three soldiers) apppeared in 1632 in Vier schöne neue Soldatenlieder (Four new soldier songs) and was later printed in Deutscher Liederhort (1856). Another version was published in 1784 (A. Elwert Ungedruckte Reste alten Gesangs “Gießen u Marburg 1784 , p. 19) and a version titled Es waren einmal drei Reiter gefangen with different lyrics was published in 1843 (Die deutschen Volkslieder mit ihren Singweisen), although it is believed to be much older.
The full song is about three soldiers who were arrested for doing something wrong. One is a young recruit. They were taken to the city to be executed, where the youngest soldier’s betrothed pleaded in vain for him to be spared. All three soldiers were executed in the market place.
The rest of the lyrics to the song can be found in German on the Volksliederarchiv website, although we can presume the place names would be different in the version sung in the mysterious land of East Holstein.
I was unable to find a rendition of the original song, but here is an instrumental version with the same melody played on the mandolin on the Folk and Classical Mandolin YouTube channel.