What mainly caused the Civil War?

[John Dibble]

Slavery in the context of state's rights was the primary reason for the Southern secession, but there were certainly other factors that contributed to it.

[John Dibble]

The Confederate states were not an independent nation as they had no right to secede in the first place.

Could you point to exactly where in the Constitution it says that states have no right to secede?

It doesn't say that there is not a right to secede, but the document's legal status makes it so secession is illegal.  The Constitution, like any other legal contract between multiple parties, cannot be voided by a singular party without that expressed right being specifically written into the contract.  Nowhere in the Constitution does it say any participating party can secede and thus void the contract that is the Constitution.  If you don't believe me, I refer you to the Supreme Court.  They ruled on several occasions that secession was unconstitutional.

If secession were not legal why did the southern states have to be readmitted to the Union?

Speaking from a legal standpoint, just because something is illegal doesn't mean people don't do it. The southern states DID secede, legal or not, so to handle it as if they didn't would be quite idiotic, don't you think?

[John Dibble]

Precisely. However, the entire basis for the "Civil" War was that the Southern states had not seceded. Therefore, to have to readmit the states into the Union was an concession that the Southern states' secession had been legitimate, meaning the entire "Civil" War was a waste.

Why are you quoting "Civil" when referring to the Civil War? "Civil" in this sense means "of or relating to citizens" - in other words, a war between citizens of the same nation. This is what most wars between citizens of the same country are generally called. Now, you might argue the Confederacy was a separate nation, but really that wasn't decided in full until the end of the war. If the South won, the war would probably be called the Confederate Revolution or something like that instead of the American Civil War, much like if the American Revolution was lost if would have been called something like "Washington's Rebellion" or "The Colonial Rebellion".