The issue of slavery was not the catalyst to war. To the contrary, the issue of slavery was later used as a political device to garner needed support for Lincoln’s drive towards a solidified union.
Slavery was the cause for secession, as the state declarations of secession and contemporary speeches by secessionists made clear. Secession was in turn the reason for war. To the frustration of abolitionists though, Lincoln, for political reasons, did not fully embrace emancipation until after the Union victory in the Battle of Antietam in September of 1862, and even then waited until after the New Year to issue it, with application limited to ten Southern states.
When Generals Fremont and Hunter suggested that early on during the Civil War the issue be made to free the slaves, Lincoln fired them.