Hitler began a campaign for global domination and genocide. The South said "Fuck this, we're out" and seceded from the Union, then the North attacked it.
The South's motivation wasn't particularly great, but their actions didn't exactly tear apart the world and change it's development forever.
People today shouldn't be ashamed of what people in 1864 did; that's silly. But they shouldn't be proud of it, either.
That's the point I think 'rayiner is making.
http://www.theatlantic.com/politics/archive/2015/06/what-thi...
That being said, I think the confederate flag is a terrible symbol to be proud of but the reaction to this game is a huge overcorrection. I have played it and it's a great game that gave me a ton of respect and insight into how pivotal that battle was. Leave it to us Americans to focus on learning the wrong lessons and applying simplistic rules to a nuanced situation.
It just so happens that their flagship "right" was the most evil and hateful thing to have ever happened systematically in America. That's why the defense of the flag falls short: in broad scope, it stands for something that really is actually kind of good. But when you take into consideration the details, it's racist as fuck.
Edit: to add on, this is the same argument as "Hitler had some good ideas". Not technically untrue, but they're far from the ideas Hitler as a figure really stands for.
And wanted to expand southwards in Mexico, Cuba and the Carribean in order to have a large slave-based empire.
https://en.wikipedia.org/wiki/Filibuster_%28military%29
If you're looking for a more serious reference, Freehling's Road to Disunion devotes a chapter to the subject.
http://www.theatlantic.com/politics/archive/2015/06/what-thi...
For extending slavery beyond the borders of south, see the quotes of Edward Pollard and Albert Gallatin Brown at the end of the post.
This is part of why the South hasn't just dropped it. The war was started, in large part, over slavery but we learned that the United states are no longer united by choice.
This is part of the ongoing argument over state's rights vs federal power. The confederate flag is a racist symbol, but it's also a rejection of federal power over the states. Politics are complicated.
I suspect the revisionist interpretation of state's rights probably comes from the 50s fight against desegregation.
It's a bit of a tautology really, but there isn't a conflict between State and Federal powers until there is a specific disagreement. The document you linked says as much:
We affirm that these ends for which this Government was instituted have been defeated, and the Government itself has been made destructive of them by the action of the non-slaveholding States. Those States have assume the right of deciding upon the propriety of our domestic institutions; and have denied the rights of property established in fifteen of the States and recognized by the Constitution; they have denounced as sinful the institution of slavery; they have permitted open establishment among them of societies, whose avowed object is to disturb the peace and to eloign the property of the citizens of other States. They have encouraged and assisted thousands of our slaves to leave their homes; and those who remain, have been incited by emissaries, books and pictures to servile insurrection.
The Confederacy (or the states that would become it) were taking over Federal military forts. The first shots fired were by Confederate troops on Fort Sumter in 1861. While the northern states (what remained of the Union) went to war, the Confederacy was not without blame in providing them with an immediate cause (Fort Sumter) to rally around.
If you're getting a raw deal politically then extracting yourself from said raw deal is probably a better option than starting a war to assert your dominance over those who disagree that your deal is a bad one.
I think the people in the South had dishonorable motivations for what I think were ultimately honorable acts. At least up until they attacked the North first.
If you read the secession declarations, the slavery question was just the straw that broke the camels back.