September 17, 1862: "America's Bloodiest Day"

[malchickiwick]

3. Making possible the Emancipation Proclamation

Though by no means a decisive victory for the North (really, it was much more of a draw), the outcome of battle nevertheless gave Lincoln enough political capital that he could order the emancipation of all enslaved human beings in those states in open rebellion against the USA. He gave them until January 1.

Also, it has been said that soldiers on their way north again in the campaign that ended at Gettysburg came across the still-unburied corpses of their fallen brothers in arms -- some nine months later!