The war had scarcely begun before slaves escaped into the Union lines. One day a Confederate officer came to Fortress Monroe and demanded his runaway slaves under the Fugitive Slave Act. General Butler refused to give them up on the ground that they were "contraband of war." By that phrase he meant that their restoration would be illegal as their services would be useful to the enemy. President Lincoln approved this decision of General Butler, and escaping slaves soon came to be called "Contrabands."

Lincoln and the Republican party thought that Congress could not interfere with slavery in the states. It might, however, buy slaves and set them free or help the states to do this. So Congress passed a law offering aid to any state which should abolish slavery within its borders. Congress itself abolished slavery in the District of Columbia with compensation to the owners. It abolished slavery in the territories without compensation. Lincoln had gladly helped to make these laws. Moreover, by August, 1862, he had made up his mind that to free the slaves in the seceded states would help "to save the Union" and would therefore be right as a "war measure." For every negro taken away from forced labor would weaken the producing power of the South and so make the conquest of the South easier.