re: If secession was legal then what right did the North have to keep the South in the USA?

“If you bring these [Confederate] leaders to trial it will condemn the North, for by the Constitution secession is not rebellion. Lincoln wanted Davis to escape, and he was right. His capture was a mistake. His trial will be a greater one.”
Chief Justice Salmon P. Chase, July 1867 (Foote, The Civil War, Vol. 3, p. 765)