What message is conveyed in the main events of “an episode of war”?

The message is that people are victims of circumstances. In "An Episode of War" by Stephen Crane, the characters are unable to exercise freedom of the will. No one can direct their own destiny (all is driven by some unpredictable fate). The only apparent certainty being the existence of human suffering.

A lieutenant gets injured in battle and refuses help (something he wasn't controlling). When doctors promise not to amputate his arm and he agrees. Then they do amputate his arm (something he clearly objected to) and his family becomes grief stricken over his missing arm.

allusion   is the right answer