sep 17, 1862 - Battle of Antietam

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Confederacy, with two victories already over the Union, wanted to win a third battle to gain foreign support from France and Britain
Union forces were aware of the Confederacy's battle plans and were able to cut Lee's army off at Antietam
Bloodiest day in the war
Lee was forced to retreat
The Confederacy's lost prevented France and Britain from getting involved
After the battle Lincoln realized that he needed to switch from an offensive war to a total war to rectify a moral wrong (began working for the Emancipation Proclamation)

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