I had at one time thought ownership of federal property was the main cause, but reecent postings to this board prove that Lincoln was content to give up forts and arsenals. Charleston was nothing special so why didn't Lincoln want to let it go? It appears to me this was Lincoln's "line in the sand" icon. Lincoln knew that if he let Charleston go the union would be divided and he was about to let half his (party) power slip away. Until the actual firing on fort Sumter the North didn't have a problem with secession, only the republican party did. As Doyle pointed out only the firing of Fort Sumter united the Union under the battle cry of Avenge Sumter.
SLAVERY HAD NOTHING TO DO WITH THE WAR.