I believe it was episode of Tom & Jerry. Can't find it.
There was episode where old mouse was talking to little mice behind school desks. He was describing war: humans had long trunks... There was war and eventually humans eliminated each other.
This is the cartoon Good Will To Men(1955). It was directed By William Hanna and Joseph Barbera. The production team was the same team responsible for the Tom and Jerry cartoons although Tom and Jerry are not in it. The "men with long trunks" were soldiers wearing gas masks. The old mouse was the chorus master and he explains to the young mice how all the men killed each other. The cartoon intercuts scenes of the chorus master explaining what happened and scenes of battle illustrating his explanation. The cartoon is available on YouTube.