Final answer:
Following the Civil War, the issue of slavery was resolved by the adoption of the Thirteenth Amendment, which abolished slavery across the United States and was a requirement for secessionist states to rejoin the Union.
Step-by-step explanation:
The federal government dealt with the issue of slavery following the Civil War primarily through the Thirteenth Amendment, which made slavery illegal in the United States. On December 6, 1865, the amendment was adopted, eight months after the end of the Civil War. Contrary to the Emancipation Proclamation, which was a wartime measure and freed only slaves in rebellious states, the Thirteenth Amendment formally abolished the institution of slavery throughout the country. Moreover, Congress made ratification of the Thirteenth Amendment a condition for the secessionist states to be readmitted into the Union. This marked the end of any form of legal slavery within the United States and its jurisdictions.