Yes, but they lost because ultimately they couldn't explain why one person should own slaves and another should be enslaved, other than luck. Rights are indeed what people assert, but to argue they have no conceptual existence is to confuse them with power relations.

No, they lost because their military power wasn't sufficient to impose their view of "rights" onto those they were fighting.

Again, we all likely agree that there is a moral imperative in play here that slavery should not be tolerated. And had the South won, I'm sure the guy being whipped against a post would be super comforted by us sitting around talking about his universal human rights. He could think about them to take his mind off the lashings.

I'm not arguing that rights have no conceptual existence. I'm arguing that conceptual existence isn't the existence that really matters. It's all lovely for me to say that anyone has the right to draw whatever cartoons they want, but there are 30 million people in Saudi Arabia who would rightly call me an idiot. The actual existence of the right is not some airy concept that I just get to define because it pleases me. It's a very practical thing that affects people's daily lives, and I don't get to declare otherwise just because it suits me.

That's certainly true once you get to the kinetic stage of war with people actually battling, but I'd back up and look at the political breakdowns that led to that outcome. It does feel like we basically agree though, and are just describing the same thing from somewhat different perspectives, so I don't want to be pedantic about it.

Here's a counter argument: Every single peasant revolt in history failed and resulted in the peasants often having even fewer rights. At the very least everyone involved in the revolt received extreme sanctions.

Revolts only started working during the enlightenment era when the idea of rights started being supported by the nobles as well. Conceptually at least. That's where we got the French Revolution, worker's rights in the 19th century, and stuff like that.

Think about it, until the 1st world war or thereabouts, we as commoners didn't even have the right to free time.

I'm inclined to agree, but I'd like to look back over more world history - not to dispute your class argument, but to point to wars of independence that function as a proxy by expanding national rights. I'm thinking of China's wars of unification and with their various neighbors, but I don't know the subject well enough to think of specific examples offhand.