President Abraham Lincoln issued the Emancipation Proclamation on September 22, 1862. It declared that as of January 1, 1863, all slaves in states rebelling against the Union would be immediately and forever free.

The Emancipation Proclamation was not a law. It was a proclamation issued by the president as Commander in Chief in time of war. Lincoln alone did not have the power to create a law concerning slavery. In fact, although he was anti-slavery, he was cautious in the wording of the proclamation because he did not want to alienate the states in the Union that still supported it. At the time, slavery was written into the Constitution, and it would take a Constitutional amendment to change it. For this reason, he worded the proclamation very carefully, making it clear that it was a wartime measure. It had tremendous influence, however, in altering the motivation of the war from a mere fight against rebellious colonists to a struggle for human freedom. In response, many slaves fled their owners in the South and joined the Union army.

Several laws enacted in the North before the Emancipation Proclamation acted as preludes to it. For instance, before the proclamation was issued, Congress passed a law that southern slaves could be confiscated as contraband. In another law in March 1862, Congress made it illegal to return fugitive slaves to their owners. These laws did not change the status of slaves, however. They only prevented fugitives from being returned. In July 1862, Congress passed another confiscation act that freed the slaves owned by rebels against the Union, and another act allowed freed slaves to join the Union Army.

Because the Emancipation Proclamation was issued as a war measure, it might have had no validity once the war had ended. For this reason, it took the thirteenth Amendment to the Constitution, which was finalized on December 18, 1865, to legally end slavery throughout the United States.