Private Peaceful by Michael Morpurgo

Tommo and Charlie Peaceful are two young men growing up during World War I. The book details their adventures growing up in their village of Iddlesleigh, taking care of their eldest brother Big Joe who struggles with learning difficulties, and hanging out with Tommo's long-time crush, Molly. After Tommo becomes heartbroken when Molly and Charlie are revealed to be in a secret relationship, the two brothers are sent off to Belgium for the war.