Eric Foner is DeWitt Clinton professor of history at Columbia University, New York. He is the author of many books, including, most recently, The Fiery Trial: Abraham Lincoln and American Slavery (2010), which has been awarded the 2011 Pulitzer prize for history.
One hundred and fifty years ago this week occurred one of the crucial turning points of the American civil war and, indeed, of American history. Not on the battlefield, although at Antietam on 17 September 1862, a Union army forced Confederates under Robert E Lee to abandon their invasion of Maryland. Rather, it came five days later, when Abraham Lincoln issued "A Proclamation" warning the south that if the war did not end within 100 days, he would declare slaves in areas under rebellion "forever free".
Like all great historical transformations, emancipation during the civil war was a process, not a single event. It played out over time, arose from many causes and was the work of many individuals. It began at the war's outset when slaves, ignoring Lincoln's insistence that the struggle was about national unity, not slavery, began to seek refuge behind Union lines. It did not end until December 1865, when secretary of state William H Seward announced the ratification of the 13th amendment, irrevocably abolishing slavery throughout the reunited nation. But what came to be known as the Preliminary Emancipation Proclamation marked a key step in this process.
Lincoln believed that union, not emancipation, was the lowest common denominator of public support for the war. But he also understood that slavery was the war's fundamental cause. Beginning in November 1861, he promoted a plan to encourage voluntary abolition in Delaware, Maryland, Kentucky and Missouri, the border slave states that remained with the union. The federal government would provide financial compensation to states that acted to end slavery, and would encourage the freed people to emigrate to Africa or Central America. (Lincoln had long believed that "colonization", as this idea was called, would benefit both races; most blacks denounced it as a denial of their right to equality in the United States.)..