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On Sept. 8, 1862, more than 40 ministers in Chicago signed a nine-page “Memorial of all Christian Denominations,” urging Lincoln to emancipate the slaves on moral and religious grounds, which two of them, the Rev. William W. Patton and the Rev. John Dempster, personally delivered to Lincoln five days later. The three had a long conversation, which the ministers duly reported back to their colleagues and which was later published in The Chicago Tribune and other papers.

In their conversation, Lincoln explained why he could not emancipate the slaves of the South, a response that illustrates his political shrewdness in moving the United States to accept the necessity of emancipation and the moral necessity of converting the war for union into a war for freedom.

The ministers urged Lincoln to emancipate the slaves because, they said, it would comport with God’s will. Never a churchgoer and not formally religious, Lincoln nevertheless understood the power of faith and the importance of having the support of religious leaders. He began by saying that he was bombarded by advice from “religious men” about “Divine will,” but that the advice was conflicting. Lincoln suggested that if “God would reveal his will to others, on a point so connected with my duty, it might be supposed he would reveal it directly to me.” But in fact he had had no divine revelation. If he did, he would certainly do what the Lord asked, but Lincoln reminded the ministers that “These are not” the “days of miracles.” Lincoln could not look to religion or the divine will to determine how to deal with slavery. He had to look to more practical issues.

Lincoln then set out the problems associated with emancipation. He asked the ministers “what good would a proclamation of emancipation from me do” given the current military situation. In a brilliant moment, he said: “I do not want to issue a document that the world will see must necessarily be inoperative, like the pope’s bull against the comet.” This was a reference to Pope Calixtus III, who, in 1456, allegedly issued a Papal Bull ordering Halley’s comet not to come to earth; Lincoln did not want to issue a proclamation that he, like Calixtus, could not enforce.

But to make sure the ministers understood his position, Lincoln clarified the point: “Would my word free the slaves, when I cannot even enforce the Constitution in the rebel states?” In other words, he couldn’t move against slavery until he was in a position to enforce his will with military power. This point turns the traditional view of emancipation upside down. Lincoln was not willing to issue a proclamation from weakness – as a desperate measure to win the war – but only from a position of strength, when he felt he could actually win the war and preserve the freedom of the former slaves.

Lincoln told the ministers he saw no legal or constitutional problems with freeing the slaves in the Confederacy in his capacity “as commander in chief of the Army and Navy, in time of war.” On the contrary, he said, “I suppose I have a right to take any measure which may best subdue the enemy.” And he assured the ministers he had no “objections of a moral nature” to ending slavery. But a premature emancipation might push Kentucky into the Confederacy and remove “50,000 bayonets in the Union armies from the Border States.”

Freeing the slaves in loyal and occupied states would lead to massive dislocations of population, force the Army to feed thousands of black refugees and distract his troops from the war effort. Nor could he arm the slaves, because they were not prepared to be soldiers and “thus far we have not had arms enough to equip our white troops”; he also worried that if black troops were captured they would be re-enslaved. Lincoln ended this meeting by assuring the ministers that the issue of emancipation “is on my mind, by day and night, more than any other.”

On Sept. 20, Patton and Dempster reported the substance of the conversation, with many direct quotations from Lincoln, to a meeting of ministers in Chicago. Doubtless, they were encouraged by Lincoln’s admission that he saw no legal problem with emancipation and that he had “not decided against an emancipation,” even as they were doubtless discouraged that he held “the matter under advisement.”

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Most of Lincoln’s arguments against emancipation seem, in retrospect, designed to encourage the ministers to publicly advocate for emancipation, rather than to discourage their support. This was doubtless part of Lincoln’s strategy to build support in the North for a full-scale assault on the institution. Lincoln’s arguments about black troops were misleading, because by this time he had already authorized his generals on South Carolina’s Sea Islands to begin training and arming former slaves.

Similarly, while expressing fear that Kentucky might join the Confederacy, Lincoln surely knew better, since the state’s secessionist governor, Beriah Magoffin, had just resigned and fled to the Confederacy. Kentucky was militarily and politically secure, but Lincoln did not need to acknowledge that at this time.

Most important, what the ministers could not know was that Lincoln had already written the Preliminary Emancipation Proclamation and gained the support of his entire cabinet to issue it at the appropriate moment. In fact, he was to release it two days after they reported back to their colleagues in Chicago.

Lincoln’s response to the ministers was part of his strategy of preparing the North for emancipation and increasing support for it. He knew where he was headed, but in mid-September, he could not reveal his plans to the ministers, and of course couldn’t know when he would be able to move against slavery.

He was soon able to make that move. On Sept. 17, just four days after the meeting with ministers, the United States Army won its first major battle in the Eastern campaign, defeated Gen. Robert E. Lee’s Army of Northern Virginia in a decisive battle at Antietam. This was the big victory Lincoln was waiting for. Five days later, on Sept. 22, he issued the Preliminary Emancipation Proclamation. The war for the Union was about to become the war for freedom.

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Paul Finkelman is the John Hope Franklin visiting professor of American legal history at Duke Law School and the President William McKinley distinguished professor of law and public policy at Albany Law School.