Answer:
Psychologically and politically.
Explanation:
The battle of Antietam was not crucial, Lincoln hoped it would be. However, the Union was now in a position to psychologically, politically and economically attack the Confederacy. In January 1863, Lincoln published the Emancipation Proclamation. The text implied freedom for all people controlled by the Union and freedom became a military objective so that as time progressed, more and more slaves were freed and joined the Union troops.
And if the proclamation did not have much resonance among Southerners, surely such a Lincoln attitude was psychological of exceptional value for the continuation of the war. The proclamation of emancipation brought with it a political victory, it was an official step towards the abolition of slavery, and the rest of the world was viewed by the Confederacy as barbarians still nurturing slavery.
