They wanted a big country. They didn't want to have a competing country a few miles away. They weren't anti-slavery at the time -- though they were moving in that direction. Nobody denies that all the original states had slavery during the Revolutionary era, but that's not what we were talking about.
You said: "You might say that in 1861 the Deep South was motivated by a desire to preserve the Constitution." I wouldn't say that, because I recognize that equating slavery with the Constitution is fallacious. Also, it's strange to say that repudiating and rejecting the Constitution is somehow preserving it.
It is not what you were talking about. Probably the reason you don’t talk about it much is because you don’t want word to get around.
The slaves states - all 13 of them - wrote slavery into the U.S. constitution. And the 13 states provided a method of changing the constitution - constitutional amendments.
Do you know why Lincoln and the northern states did not use the constitutional amendment process to end slavery peacefully?
“They wanted a big country. They didn’t want to have a competing country a few miles away.”
Agree, but there is more. They wanted secure borders. They wanted more allied states in the event of war. They wanted trading partners and a larger economic market. They wanted, as you say, a big country, to produce prosperity and wealth.
They wanted, in a word: money.
And that’s why northern states wrote slavery into the constitution - until their accountants convinced them they could make more money with a different workforce model (combined with monopolies).
Ten seconds after that, they found that it was morally wrong for economic and political rivals in the southern states to own slaves.
Over simplified? Yes, but not by much.