The South expected massive cotton revenues to prop its economy.

https://en.wikipedia.org/wiki/King_Cotton

To be fair however, it was known how the war would proceed before the outset!

In the words of General Sherman:

You people of the South don't know what you are doing. This country will be drenched in blood, and God only knows how it will end. It is all folly, madness, a crime against civilization! You people speak so lightly of war; you don't know what you're talking about. War is a terrible thing! You mistake, too, the people of the North. They are a peaceable people but an earnest people, and they will fight, too. They are not going to let this country be destroyed without a mighty effort to save it… Besides, where are your men and appliances of war to contend against them? The North can make a steam engine, locomotive, or railway car; hardly a yard of cloth or pair of shoes can you make. You are rushing into war with one of the most powerful, ingeniously mechanical, and determined people on Earth — right at your doors. You are bound to fail. Only in your spirit and determination are you prepared for war. In all else you are totally unprepared, with a bad cause to start with. At first you will make headway, but as your limited resources begin to fail, shut out from the markets of Europe as you will be, your cause will begin to wane. If your people will but stop and think, they must see in the end that you will surely fail.

People are not rational however, and the wealthy landowners were facing the extinction of their wealth... so they started a war.

> the wealthy landowners were facing the extinction of their wealth... so they started a war

As someone who doesn't know much about American history: What happened to the wealth of those Southern landowners after the war? Did they get to keep it?

Some of it. Many of those who survived lost a good portion of their capital. But they kept title to their land, and used that leverage, voting power, and racist solidarity over time to claw back much of what they had previously enjoyed from exploitation of the emancipated slaves.

The South was still devastated by the war. But being largely agrarian to begin with, loss of capital wasn't quite as hard a blow as losing a factory.

Much of the "wealth" was the people they enslaved, so they did not get to keep it. The American Civil War is seen as a huge and enduring economic loss (but moral gain) for the southern states.

The people driving that were very wealthy.