[MoSattler]

This illustration was created shortly after the American Civil War began, a conflict that started when the South seceded.

[sirobg]

Thanks! I think I have the context, but I don't get the joke

[UncleSlacky]

The oil wells were in Pennsylvania, which was on the Union side.

[sirobg]

So the whales are saying: "May They Never Secede" talking about the wells, in reference to the confederacy whom already seceded. And they say this because obviously if the wells "seceded", humans would go back to whale hunting.

Do I understand it correctly now?

[fzzzy]

Yes.