Sorry, but the decision for war was not Lincoln's, it was Jefferson Davis' -- when Davis chose to start war at Fort Sumter and then soon after formally declared war on the United States.
Lincoln's choice was simply victory or defeat, and he chose victory -- since that is what the Constitution expects when the United States is attacked.
Lincoln also expected that a Union defeat would mean the loss of every slave-holding state and territory and the likely dissolution of the remaining "United States".
By the way, in 1860 US politicians had known for decades that a Civil War would necessarily destroy slavery wherever Union armies controlled slave-holding territory.
Military precedents were set in previous wars, and learned opinions given by the likes of former President John Quincy Adams.
So it would be fair to say that though slavery's destruction was not a stated goal at war's beginning, many Union leaders understood at the time that slavery would not survive a Union victory.
It was Lincoln’s decision whether to allow the peaceful secession to stand or whether to retain the South by force. His insistence on maintaining Union forts within the territory of the former states cannot indicate anything other than that he intended to retain them by force. The Confederacy may have fired the first bombardment, but Lincoln was strolling down the path of war by refusing to abandon Sumter once South Carolina left the Union.