The Emancipation Proclamation was primarily a military measure, freeing only those slaves held in the Confederate States. President Lincoln was never sure the proclamation was constitutional, and he feared that the next president could overturn it.
He urged Congress to abolish slavery by constitutional amendment, depriving Southern slaveholders of $2 billion in human property and ensuring the permanent freedom of more than 4 million in bondage.
Congress sent the proposed amendment to the states for ratification and after the Civil War ended, President Andrew Johnson required that all former Confederate states adopt the amendment before rejoining the Union. Georgia became the 27th and deciding state to ratify it, and Secretary of State William Seward declared the 13th Amendment ending slavery officially part of the Constitution on December 18, 1865, Today in Georgia History.
Mississippi was the final state to ratify the 13th Amendment in 1995.