Sorry, wrong.Lincoln pulled a Athens and forced war where none was legally justified because he wanted to keep the empire a empire. Allowing vassals to secede would mean dissolution of empire and the loss of power for Washington..... Lincoln might not have like to call it that but he was just as much an imperialist as Churchill.
Slavery is something that entered in after the fact.
Also like I said, it's about even handedness.
The Southern States succeeded because they wanted to maintain slavery as an institution, after the mostly Northern states began moving to abolish it.
Slavery as a reason for the succession was cited by all states that succeeded, i.e. before the war.
This led to the war. Lincoln was President of the USA and fought to defend the Union.