Where did you think they got the name "Republican Party" from? They didn't draw names out of a hat. The name reflects their core values including that the US should be a republic and not a democracy or federation. In a Republic states rights come second and that includes the right to secession. Lincoln was their first president and fought the bloodiest war in US history to ensure that the term "Republic" stuck for good. Republicans may be today's conservatives who espouse states rights, but originally they were the radical upstarts who promoted taking away states rights such as slavery!
I'm very sympathetic to Lincoln's position about the south's secession. And in terms of building the US into a global power - arguably a world champion for freedom and democracy - it was a great success. I like a lot about the result of his policy - there are all kinds of benefits.
Yet there is the principle of self-determination - I have my opinion as a Californian, but the South has its opinions about what they want.
I abhor their immoral position on slavery - which has its irony as they grossly deprived a group of their right to freedom and had their 'freedoms' denied as well.
But in terms of the basic question whether they should be free to not permanently be part of the US if they don't want to, there's still the issue of that freedom.
While they had no defense on their moral crimes on slavery, they did have legitimate grievances against the federal system for having 51% be abusive to them.
Grievances there's little they could do about under our system.
The next state might have far better reasons, different than slavery, and their freedom will be impaired. The United States is an amoeba now slowly gobbling the world.
Of course, every other country without the right to secede is as well, leading to the inevitable conflicts as one amoeba eats another.
I think the essential thing for freedom is to somehow maintain diversity of power in the world - the opposite of the current agendas of the most powerful nations.
We've had some luck as the 'right' amoebas won previous battles, as Germany and Japan did not get to expand their empires in WWII for long.
But since then we've seen every powerful nation involved in this aggressive agenda as millions have been killed, tortured, and more, often by 'puppet' dictators.
Self-determination did not happen for those less powerful countries and people paid heavily.
It's never easy to deal with these things - what granularity has the right to secede? the Kurds in Iraq? A county or city in the US? The Chechnyans or the Tibetans?
But the basic idea of self-determination is one we should try to support - such as the recent developments in the Middle East, opposing our dictator Mubarak et al.
Why don't we give the states reasons not to secede instead of guns telling them they can't?
How could the civil war conflict have gone if the North had been fairer on issues other than slavery?
It worked out well on slavery, ending it - even if that hadn't been the intent, for example Lincoln having plan to phase it out by 1800 - but there were other issues.