Lincoln is believed to have started working on an emancipation policy in the summer of 1862. In late July of that year, he presented to his cabinet his plan to proceed with ending slavery in states in rebellion against the Union. ...
Secretary of State William Seward suggested that Lincoln wait for a military victory before publicly issuing the proclamation, to avoid having it appear as an act of desperation. Five days after the Union stopped Lee's army at Antietam, Maryland, the president issued the preliminary emancipation proclamation on September 22.