Battle of Antietam

" From a tactical standpoint, the battle was somewhat inconclusive; the Union army successfully repelled the Confederate invasion but suffered heavier casualties and failed to defeat Lee's army outright. However, it was a significant turning point in the war in favor of the Union due in large part to its political ramifications: the battle's result gave Lincoln the political confidence to issue the Emancipation Proclamation, declaring all those held as slaves within enemy territory free. This effectively discouraged the British and French governments from recognizing the Confederacy, as neither power wished to give the appearance of supporting slavery." - https://en.wikipedia.org/wiki/Battle_of_Antietam