On September 22, 1862, President Lincoln had issued a preliminary proclamation that he would order the emancipation of all slaves in any state (or part of a state) that did not end their rebellion against the Union by January 1, 1863. None of the Confederate states restored themselves to the Union, and Lincoln’s order, signed and issued January 1, 1863, took effect.
It was published in the National Republican the next day.
The announcement came a week after the Union victory at Antietam on September 17th. On that day the Union had 12,401 casualties with 2,108 dead. Confederate casualties were 10,318 with 1,546 dead. This represented 25% of the Federal force and 31% of the Confederate. To this day, it is the deadliest day in American history.
Lincoln’s cabinet had told him to hold the announcement until he had a victory to precede it and Antietam gave him the opportunity to move forward. The announcement turned world opinion in favor of the Union, foreclosing a European power from providing a lifeline to the rebels as they had done a century earlier.