One of the most important documents in US history - a proclamation that freed slaves in the secessionist South - will go under the hammer with a targeted sales range of $1-1.5 million, Sotheby's of New York said Wednesday.

Lincoln document that freed slaves now on sale

The copy of the Emancipation Proclamation being sold Dec 10 was signed by president Abraham Lincoln Jan 1, 1863, in the midst of the Civil War - part of a series of actions that eventually freed all US slaves.

The document, one of 19 known surviving copies, is being sold by the family of the late Robert Kennedy, who was assassinated during his presidential bid in 1968. Kennedy, whose brother president John F. Kennedy was also assassinated, helped push through civil-rights legislation in the 1960s as a US senator from New York.

At least 14 of the copies are held by institutions. On its way to the auction block, the Kennedy copy will be exhibited in Boston, Philadelphia and New York.

Lincoln took office in 1861, just as the Civil War began. He was determined to prevent the spread of slavery beyond the US South and to force the rebellious Confederate states back into the union.

In 1863, Lincoln issued what is known as the Emancipation Proclamation, freeing slaves in the 10 Confederate states that were still in rebellion. Six other states practicing slavery that had remained in the union or been recaptured were omitted from the proclamation.

Slavery was not permanently abolished throughout the US until the 13th Amendment to the US Constitution was adopted in December 1865. Lincoln had already been assassinated by a Confederate sympathiser in April 1865, days before the end of the war.

It would take another 100 years, including the civil rights movement and another set of legislative reforms to dismantle the apartheid system that was re-established in the South in the late 1800s. The 1964 Civil Rights Act and 1965 Voting Rights Act helped guarantee African-Americans equal protection under the law and full rights in US society.

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