Ok, slavery wasn’t a significant part of the Union cause at the start of the war. However, there had already been huge political battles over it between north and south before the war (compromise of 1850, anyone?). Also, the Emancipation Proclamation kept England and France from recognizing the CSA, which kept the CSA isolated, and allowed the Union to bludgeon it with superior numbers and production.

Yeah. Slavery wasn’t important at all.
/