Do you really believe that the Founders would allow states to leave in a way guaranteed to cause disagreement and conflict?
It wasn't guaranteed to cause conflict. Disagreement, sure, but conflict, no. They had hoped that the states if necessary could just part in peace and there would be no conflict. The only reason there was conflict was because the North refused to simply let the South go in peace, just like Great Britain would not let the colonies go peacefully. Both stood to lose too much economically. They didn't care about the rights of the other states.
If they thought that was an option under the Constitution that they ratified then it turns out they were mistaken.
And as we know, a right that one state has is a right that all states have.
And a restriction one state has is a restriction all states have. The states are co-equal. No one state has more rights than another.
Not sure where you are getting the idea that a state can't leave a union that they voluntarily joined.
And again, with the exception of the first thirteen states, none of the states "joined". They were admitted with the permission of the existing states, and only once they received that permission. That is all that matters, permission to join not desire to join. There are several cases of states, like Colorado, who wanted to join long before they were allowed in.
If France were to leave the European Union, should all the other countries wage war against them to make them stay, in order to "preserve the union?"
Well I don't know. Is the European Union goverened by the Constitution of the United States?
It wasn't guaranteed to cause conflict. Disagreement, sure, but conflict, no.
Well let's see. They walked out without discussion. They walked away from any responsibility for debt or treaty obligations the country took on while they were a part. The walked away with every bit of government property they could get their hands on. Seems to me that conduct like that was guaranteed to lead to more that simple disagreement.
They had hoped that the states if necessary could just part in peace and there would be no conflict.
Because they believed that any such parting would be mutual and done after both sides negotiated away any possible disagreements. I cannot find a single writing from any of the founders that says, or even implies, that a decision to leave should be one-sided and done without any thought to the impact the decision might have on the states remaining.
The only reason there was conflict was because the North refused to simply let the South go in peace, just like Great Britain would not let the colonies go peacefully.
It had been pretty peaceful from the time the states announced their secession up to the point where the South blew up Fort Sumter. So it's not that the North wouldn't let them go in peace, the South chose not to leave in peace.
Both stood to lose too much economically. They didn't care about the rights of the other states.
From a purely business and economic standpoint, an independent Confederacy would have had almost no impact on the rest of the U.S. The Confederacy would have continued to sell the U.S. cotton, the U.S. would have continued to sell the Confederacy manufactured goods, handle their finance, and broker and ship their exports because the South had no real alternative. Which is why opposition to Southern secession was political in nature, and that would have faded over time had the South not attacked.