[MUSIC PLAYING]

NARRATOR: The South had four million slaves who were vital in sustaining the Confederate war effort. Although Lincoln had always said a president had no constitutional right to attack slavery, the war had changed all that. As commander in chief, he could act out of military necessity.

MICHAEL BURLINGAME: It doesn't represent a change in his own morale. He says this is a war measure because slaves are essential for the South's war effort. If we can encourage slaves to leave the southern lines and come to our lines, we will reduce their capacity to make war. And it also comported with his own wish to see all men everywhere free.

NARRATOR: In an office of the war department, Lincoln asked for some paper, saying he wanted to write something special. He then sat down and began writing the Emancipation Proclamation.

[VOICES HUMMING]

VOICEOVER: "He would look out the window a while, then put down a line or two, and then sit quiet for a few minutes." Major Thomas Eckert, war department.

NARRATOR: Lincoln wrote this way for several weeks, adding and revising each day, asking Major Eckert to keep it under lock and key. By late July, Lincoln was ready to show it to his cabinet. The proclamation said that, in any states still in rebellion by the start of 1863, all slaves would be declared forever free. Lincoln was careful not to upset the slave-holding border states. The proclamation did not free slaves in Union-held areas, but it did encourage southern slaves to flee the plantation.

HAROLD HOLZER: He was as brilliant a public relations strategist as he was a political and military strategist. And he wanted that emancipation to be couched for the American people, in terms of saving the Union. Because he knew that issuing it as a gesture toward eventual equality-- or certainly toward abolition-- was going to be very unpopular, and could bring down the northern government.