The issue of slavery was present in national politics from the very beginning of the nation. In 1820 it was the subject of the Missouri Compromise, a measure enacted by Congress to prohibit slavery north of the state of Missouri. In the 1850s the slavery issue further divided the nation along regional lines. For the most part, however, both proslavery and antislavery positions included antiblack attitudes. Except for the abolitionists, most Northern opinion was more concerned with the dangers slavery posed to free labor than with the moral issue regarding the violation of the human rights of those held as slaves.

When the South seceded (1860-61) because of the dangers to slavery it perceived in Lincoln's election, the North declared that it was not slavery but the act of secession that precipitated the Civil War. President Lincoln supported a Constitutional amendment that would have given federal protection to slavery in the Southern states. On his order slaves who escaped into the Union lines were returned to their owners by federal troops early in the war.

Later, as the cost of the war in men and materials mounted and national support for abolition grew, President Lincoln shifted his position. In 1862 his Emancipation Proclamation declared slaves to be free if the areas in which they were held were still in revolt against the Union on Jan. 1, 1863. Slaves within the Union and in areas of the Confederacy under Union control, however, were initially excluded from the provisions of the proclamation. Thus at its inception, the proclamation functioned principally as military propaganda: slaves were declared free only in those areas where no real authority existed to free them. In those areas under federal authority, no action was taken. Nevertheless, the Emancipation Proclamation represented a point of no return on the issue of slavery.

As the war moved into various parts of the South, the actions of African Americans demonstrated the falsehood of the Southern claim of a satisfied slave population. Information and provisions were turned over to the Union troops, and slaves fled into the lines of approaching Union armies in such numbers as to create logistical problems. Letters and diaries of slave owners and their families contain frequent references to increased difficulty in controlling slaves as the fighting neared.

Beginning in 1862, provisions were made for enlisting blacks into the Union army. They were organized into all-black units referred to as the U.S. Colored Troops. Of the 209,000 blacks who entered service, 93,000 came from Confederate states. Units composed of soldiers from this area included the 1st and 3d Louisiana Native Guard and the 1st South Carolina Volunteers. The Confederacy at first refused to recognize blacks as soldiers. Unlike other Union troops who were captured, black soldiers were at first not allowed to surrender, and many were shot. The most infamous of such occurrences was at Fort Pillow, which fell to Confederate troops under Gen. Nathan B. Forrest (later a founder of the Ku Klux Klan).

African Americans took part in more than 200 battles and skirmishes. In all, 68,178 died in battle or as the result of wounds or disease during the war. Lower pay for blacks and other forms of discrimination were common. In spite of this, desertion among blacks was more than 50% lower than for the Union army as a whole.