Singing seems like a weird thing for military men to do together in the modern age of TikTok dances and faux manliness, but it used to be very common, especially when alcoholic beverages were made available.
These song books were handed out to Marines during World War II. The images contained herein are from the book given to my grandfather in 1944. As a matter of fact, you'll see his signature at the top of one of the pages.
The Fighting Fourth...