The southern perspective on the War of the Great Rebellion is not valid. It was found to be invalid by supreme court case ‘Texas v. White’.
Secession would be legal, if pursued by constitutional amendment, or by federal law, or by supreme court case. It could be de facto valid if the slave power had won their war, and managed to wrangle a peace treaty, signed by the President, and ratified by 2/3rds of the Senate, but none of those methods but the last were achieved by the slave power.