They received no such assurances. It was the states that were ratifying, not the other way around. And in each and every ratification document was wording to the effect that they agreed to the Constitution as passed out of convention in Philadelphia on September 17, 1787. And the states could add any assumption they wanted to their ratification documents and if the Constitution did not allow for it then their assumptions were meaningless.
This was not honoured obviously by Lincoln.
One does not honor illegal actions like rebellion.
The CSA / South did not “start” the war. Lincoln & the North purposely started the war when he invaded the sovereign CSA & even instigated the war when he attempted to rearm the former Union forts within the CSA thereby giving Lincoln the excuse he required when his troops were resisted.
So which did Lincoln do? Start the war? Or instigate it? Make up your mind, please.
Sumter was the property of the federal government. Built with federal funds on land deeded to the government free and clear by act of the South Carolina legislature. South Carolina had no legal claims to it whatsoever, and if the federal government merely insisted on retaining ownership of what was their's to begin with then there is nothing illegal in that. The South chose war when they chose to bombard the fort into surrender after failing to starve the garrison out.