What document changed the purpose of the Civil War from one of states' rights to that of slavery?

the Thirteenth Amendment

Declaration of Independence

Emancipation Proclamation

the Fugitive Slave Law

The answer is the Emancipation Proclamation. Technically, the war was never about ending slavery, and President Lincoln expressed that he wanted to save the Union before he freed the slaves. The Emancipation Proclamation also didn't apply to the south because they had broken off and formed a different country, so US laws did not apply to them.