in the prologue what is evidence is there to prove that the people have just lived through a devastating war?

This short parable that opens the play also sets up the structure of the play. There are two disputing parties, the goat-herders and the fruit farmers. Each group wants to claim the valley. However, the goat-herders have the claim that they were there first and should therefore keep the land, whereas the fruit farmers argue that they could put the land to better use. The Delegate moderating the debate chooses the fruit farmers because it is more logical for the person who can put the land to better use to get it.

This entire prologue is extremely Communist in its message. Any capitalist society would argue that whoever originally owned the land should get it. Brecht instead argues that whoever can best use the land should get it. It is because of the Communist overtones in the prologue that Brecht originally did not allow the prologue to be printed while he was living in the United States.

The prologue serves yet a third function of allowing Brecht to present his ideas before the play even starts. This is extremely clever of him because the audience receives the moral of the play without even having to watch it. Thus, he gets his Communist message across immediately and only after he has presented the message does he actually allow the play to begin.