Why was an amendment to end slavery necessary

Passed by Congress on January 31, 1865, and ratified on December 6, 1865, the 13th amendment abolished slavery in the United States and provides that "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction

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answer; the answer to the first question is (king james ii was replaced by his daughter mary and her dutch husband william of orange;