Final answer:
The Battle of Antietam in 1862 provided President Abraham Lincoln the Union victory he needed to issue the Emancipation Proclamation, transforming the Civil War into a moral crusade against slavery.
Step-by-step explanation:
The student is likely referring to when President Abraham Lincoln waited to issue the Emancipation Proclamation until after a significant Union victory—a strategic move to ensure it was seen as an act of strength rather than desperation. The Battle of Antietam, fought on September 17, 1862, was the bloodiest single-day battle in American history and provided this opportunity. It ended the Confederate Army of Northern Virginia's first invasion into the North and led to Abraham Lincoln announcing his preliminary Emancipation Proclamation shortly thereafter.
The battle itself could be considered a tactical stalemate, but because the Confederate troops retreated to Virginia, the Union claimed it as a strategic victory. It was after this battle that President Lincoln felt confident enough to publicly take the significant step of issuing the Emancipation Proclamation, which declared the freedom of slaves in the rebelling states. The Proclamation fundamentally transformed the character of the Civil War, as it added moral force to the Union cause and put international public opinion strongly on the Union side.