"The withdrawal of a State from a league has no revolutionary or insurrectionary characteristic. The government of the State remains unchanged as to all internal affairs. It is only its external or confederate relations that are altered. To term this action of a Sovereign a 'rebellion' is a gross abuse of language."
President Davis, CSA
The secession became a rebellion when they shelled Fort Sumter. And Lincoln reacted in the only way he could.
If the South had not shelled Fort Sumter, there would have been a good chance there would not have been war.
But too many Southerners thought they were Napoleon, Genghis and Alexander in overalls, and they wanted war.