Megiddo's Shadow by Arthur Slade.
About Megiddo's Shadow: In 1918, sixteen-year-old Edward Bathe lies about his age and joins the army to avenge the death of his brother.
Surprising and surprisingly sad. You may want to keep the tissues handy. Full review: Megiddo's Shadow by Arthur Slade
This was book #8 for the 2nd Canadian Book Challenge.