The correct answer is issuing the Emancipation Proclamation.
The Emancipation Proclamation became law on January 1, 1863. The goal of this law was to free slaves within the "rebellious states" (aka the Confederacy). Even though this doesn't technically free slaves since the Confederacy no longer followed the laws put in place by the Union, it turns the tide in the war. This war is no longer about preserving the Union. Rather, the goal of this war is to free slaves and end the institution that helpd spark this Civil War. This allowed thousands of slaves who escaped the Confederacy to join the Union army.