How does Max's father save Hans Hubermann's life in the First World War?

Hans was a mediocre 22-year-old soldier fighting in France in World War I. He was not particularly eager to fight. Hans befriends a German Jew named Erik Vandenburg, who teaches Hans to play the accordion. The day the platoom is to go into battle, the Sergeant asks which one of them has good handwriting. None volunteered. The Sergeant says that whichever man does will not be going into battle, yet no man wants to seem like a coward. Erik nominates Hans, who is sent to write letters for the captain. The rest of the men are all killed.