how does lepers telegram destroy "separate piece"?

The boys are celebrating the winter carnival. They are truly insulated from the harsh realities of war. The carnival is a place of innocence and happiness. A telegram comes for them from Leper; he says he has "escaped," and needs them to meet him and help to bail him out of whatever trouble he is in. Finny and Gene are absolutely shocked that their friend has deserted, and are determined to meet him and help him out. This shock of reality intrudes on their care-free winter fun.