If the Civil War had begun as a war to restore the Union, how did this Proclamation changed the purpose of the war?

The Proclamation changed the purpose of the war into one of freeing slaves completely, making the long war into one about wether or not slavery would remain in place . The South claiming that they needed slaves to work on their plantations and needed slaves to help around was a major part of the war . They wanted to continue on this racist act of holding “different” people under their feet. Many say now that the this was not a reason for the war nor the separation between the north and south but when looking back at every statement made by every southern state slavery is mentioned for their uprising .

How are we suppose to know this