Final answer:
The Thirteenth Amendment to the United States Constitution is credited with ending slavery in the United States, becoming law in December 1865.
Step-by-step explanation:
The document credited with ending slavery in the United States is the Thirteenth Amendment. President Abraham Lincoln, realizing that an emancipation proclamation would not have the permanent force of law, supported the movement for a constitutional amendment to abolish slavery.
After his assassination and the conclusion of the Civil War, the movement saw success. The Thirteenth Amendment was passed by Congress in early 1865, ratified by the states, and by December 1865, it became law. This landmark amendment declared that neither slavery nor involuntary servitude would exist within the United States, effectively ending the legal practice of slavery.