[Colonel Kangaroo]

The War of 1861 was not a civil war since the South had no interest in taking over the North—they just wanted to be left alone.

If that's the case, why didn't they just stay in the Union? Lincoln himself said he did not want to intervene in the Southern states and no better framework existed for inter-regional cooperation than the US Constitution.

[AlanD]

The truth of the matter is that the South wanted to be left alone so that is could extend it’s slave territories to the South . . specifically, Nicaragua, Cuba, and Mexico.

The South knew that the North would never allow that to happen, so that secession was necessary.