What happens to cause the lieutenant's injury?

As often happens in wartime, injuries can be caused at the least expected times. Such is the case in Stephen Crane's short story, "An Episode of War." The lieutenant is busy rationing coffee for his men during a break in the fighting when he suddenly cries out. Thinking a fellow soldier has assaulted him, he looks to the nearest man. But the rest of the nearby soldiers quickly voice their concern, and when the lieutenant looks at his arm, he finds blood trickling down his sleeve. The lieutenant "winced... swayed dangerously, and then straightened." He looks to the woods in hope of seeing from where the shot had come while the men stare "silent, astonished and awed by this catastrophe which happened when catastrophes were not expected--when they had leisure to observe it."