The Emancipation Proclamation. James M. MacPherson's New York Review of Books review last year of four new scholarly works on the subject, "A Bombshell on the American Public," explains why. Among the reasons:
The proclamation officially turned the Union army into an army of liberation—if it could win the war. And by authorizing the enlistment of freed slaves in the army, the final proclamation went a long step toward creating that army of liberation. If the Emancipation Proclamation was merely a piece of paper that did not actually free anyone, as skeptics then and later charged, the Declaration of Independence was likewise a mere piece of paper that did not in itself create a new nation. Both outcomes depended on victory in a war to which these documents gave new purpose.