Slaves contributed directly to ending slavery by joining Union forces as they moved through the Southern states. Union generals treated the slaves as contraband and often put them to work creating roads and digging ditches. Some Union commanders, such as William Sherman, viewed the slaves who left the plantation as more mouths to feed and a drain on the army's resources, but the slaves provided valuable labor to the Union army. Slaves also provided information on Confederate forces and troop strength. Many Southern planters viewed the slaves as too dumb to know this information or too loyal to give up this information to the enemy; these attitudes towards slaves would hurt the Confederate cause. This was especially true during Sherman's March to the Sea, as Union forces encountered Confederate guerrillas south of Atlanta.

Slaves contributed indirectly to the end of slavery as well. There was a continuous threat of slave revolt in the South—this threat was heightened by the Emancipation Proclamation. In order to keep slaves at home working on the plantation, many Southern men stayed home in order to manage the slaves. The South had a manpower shortage when compared to the Union army. Just by their existence on the plantation, slaves pulled men from serving in the Confederate army.