Slavery was a key issue during the Civil War time period—it was the most critical issue influencing secession. Throughout the 1850s and 1860s, slavery and its continued survival was a critical and divisive question within the United States, and as the country expanded toward the Pacific, slavery's legality in the territories proved a heated point of controversy. We can trace one flash point after another: between the Great Compromise of 1850, the publication of Uncle Tom's Cabin by Harriet Beecher Stowe, the Kansas-Nebraska Acts, the Dred Scott Decision, the Lincoln-Douglas Debates, and John Brown's attack at Harpers Ferry, the United States was becoming a political battleground between pro-slavery advocates and abolitionists. This was a struggle struggle which neither side felt they could afford to lose, and as we came closer and closer to the Civil War, both sides became more and more entrenched, until we hit the election of 1860, when Lincoln was elected president and Southern states ultimately responded with secession.

Slavery continued to be a major issue and influenced the outcome of the Civil War itself. While the primary objective of the Union remained to restore national unity, it must be remembered that abolitionism was a profound and powerful moral position. With this in mind, the Emancipation Proclamation (which banned slavery for all states in rebellion) proved a profound turning point in the Civil War and introduced an additional moral component into the struggle—it made the Northern cause one aimed at ending slavery within the United States. In doing so, Lincoln isolated the Confederacy politically (with their cash crop economy, the Southern states would have had a natural ally in Great Britain, but the British were opponents of international slavery). The Proclamation also had a significant impact for the Union's recruitment effort (particularly among black populations). It was a key turning point of the Civil War and was critical in changing the face of that struggle.