However seceeding was not really a rebellion. It was a withdrawal. Much like what would happen if we withdrew from the UN. Would you consider withdrawing from the UN a rebellion?
In the 14th amendment where they talk about rebellion a lot, what they are referring to is the confederacy. They were saying that the US would not be held responsible for the debts of the confederacy and that politicians that led the confederacy would not be permitted to take seats in Congress. You could maybe argue that the reference to rebellion in the original constitution wasn't meant to include succession, but definitely the post 14th amendment constitution does include succession as rebellion.
Also, I think the confederacy considered it to be a rebellion too. That's where "rebel yell" comes from.
A rebellion is something happens when you attempt to take over an existing government. The southern states was not attempting to do this. They were perfectly willing to let the rest of the Union be as it was.
Succession is basically overthrowing the government in one part of the country.
However, the South was not a part of those preceedings. Remember, the South seceeded from the Union. As such those states that were still a part of that Union did not include the southern states in the Constitutions representitive status. Indeed, the southern states had to reapply for admission into the Union before they were again allowed a say in Congress/Senate and to be considered a part of the United States. Knowing that they had to reapply shows that the US at the time did consider the south to not officially belong in the Union. Instead they were considered territories. To apply a present day comparrison think of the Virgin Islands. Only difference being that with the Southern states we had to conquer them. We bought the Virgin Islands.
Yeah, that's right. Although, it depends how you mean "not officially belong in the Union". Territories aren't outside of the sovereignty of the US, they're just denied local government. They were ruled by federally created military government. They were certainly not independent nations, they were subjugated by the US.
Of course, once the southern states reapplied for admission into the US they then accepted all the amendments, including the 14th.
They were more or less forced to. The federal government refused to allow them to form a civil government or participate in government at the federal level until they agreed to the 14th.
Which would apply if the Southern states had violated any of the laws set down by the US Constitution or any other law of the time. However AFAIK there was nothing in the Constitution which prohibited secession (there still isn't) and there was no law outside of the Constitution which also prohibited secession.
For most things, if the constitution is silent, that means it is permitted. But succession isn't most things. Succession is refusing to adhere to all the other commitments made in the constitution. The states signed off on granting all kind of authority to the federal government. If that was not intended to be binding on them, the constitution would have to say that explicitly- that succession is an exception to all of the above. But without that, the language is clearly binding.
On top of that you have the harsh treatments of rebellion, insurrection, treason, and making war against the United States sprinkled throughout the constitution. Article 3, section 3 says "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort ... The Congress shall have power to declare the Punishment of Treason". Article 1, section 8 says that Congress can "provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections". Article 1, section 9 says that "The privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it." These three taken together mean that succeeding and declaring war on the US is a crime which Congress can punish, that the military can be used to suppress succession attempts, and that all legal protections can be denied those who succeed. That was all in the constitution before the civil war. Nowdays, with the 14th, it is even more explicit.