President lincoln issued the emancipation proclamation after seeing the union’s effectiveness at the battle of antietam.
realizing the south could win and slavery would not end.
seeing general mcclellan lose at antietam.
watching general lee capture washington, dc.

The Emancipation Proclamation ended slavery and set all slaves free.
It was signed on September 22nd, which was soon after the Union's victory against the Confederates in the Battle of Antietam.
It was this victory that prompted President Lincoln to pass the Emancipation Proclamation.

So the answer is A. Seeing the Union’s effectiveness at the Battle of Antietam.

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