PTSD

[Mitra]

"Come home with your shield or on it"

This refer to survive after a lost fighting; this happen frequently to the loser of battle of other greek cities, but they are not take as deserters they return to home. The spartans don' admit the defeat as possibility. The deserter is another concept.

As for deserting during battle I understand it wasn't uncommon during the US Civil War as well as the First World War.

These are different type of wars, where the armies cover much space, have a great number of men, battles are separated by pause moments, and the confusion and the rumour are enormous. Nothing of comparable of a minor fighting like that of letter.