I like to think that preserving the country that our Founding Fathers bequeathed to us, whole and intact, was high moral cause enough.
"African slavery, as it exists in the United States, is a moral, a social, and a political blessing."~~~ Jefferson Davis
“Whole and intact”?
Not quite whole and intact. It was missing something. About 650,000 people, many of them descendants of the Founding Fathers.
And it lost the original constitution bequeathed by the founding fathers.
Arguably, it was a small price to pay to be happily ruled by Washington D.C.
No more "moral" than preserving the United Kingdom according to your proffered arguments.