Disunion follows the Civil War as it unfolded.

On July 22, 1862, Lincoln convened his cabinet to reveal a preliminary draft of the Emancipation Proclamation. Lincoln informed the men that he “had resolved upon this step, and had not called them together to ask their advice, but to lay the subject-matter of a proclamation before them.” He did, however, open the floor for discussion.

For some, it was the first they had heard of the plan; for others, like Secretary of State William Seward and Secretary of the Navy Gideon Welles, it was a secret Lincoln had let them in on during the previous week. Still, it was a bombshell for all involved: the culmination of weeks of antislavery activity on Capitol Hill and in the White House, a brief period of time that would reset the purpose of the war and, in doing so, put the nation on a fundamentally new course.

Lincoln decided to issue the proclamation after months of unsuccessful pleading with leaders from the loyal slave states to phase out the institution of slavery over time in return for federal compensation. He hoped that such an experiment, if successful, might show the Confederates how to rid themselves of the evil institution later on. Lincoln now decided to reverse this sequence: he would free the slaves in all the rebel states first, and then deal with the border states later.

In the midst of all this, the Republican Congress passed a crucial new piece of legislation: the Second Confiscation Act, which emancipated slaves of individual rebels in Confederate states. The act was immensely controversial all over the North: white supremacists were up in arms. Lincoln worried that a huge white backlash might give pro-slavery Democrats control of Congress in the mid-term elections that were coming in November. And he also had concerns about the constitutionality of some sections of the bill. So he prepared a veto message. But the message suggested a number of remedies that would make the bill acceptable. For example, he wrote that “it is startling to say that congress can free a slave within a state” — but “if it were said that the ownership of the slave had first been transferred to the nation, and that congress had then liberated him, the difficulty would at once vanish.”

Lincoln’s legal skills and political acumen strengthened the Confiscation Act. To ensure passage, Lincoln met secretly with members of Congress to craft a definitive compromise. The newly revised bill passed, and Lincoln signed it on July 17. Five days later, he convened his cabinet.

In the meantime, however, he had drafted a document immensely more radical than anything Congress had considered. The radicalism of Lincoln’s new idea came down to the fact that he made no distinction whatsoever between the slaves who were owned by rebels and any other slaves. Under Lincoln’s plan, all slaves within the states or the parts of states where rebellion existed would be freed just as soon as Union armies could free them.

Lincoln presented his proclamation as a putatively military measure. It provided that all slaves in Rebel-controlled territory were thereafter free, a move that exempted both the loyal slave states (Delaware, Maryland, Kentucky and Missouri) and the Union-held portions of Confederate states, primarily Tennessee and Louisiana. Of course, the document meant nothing, immediately, for the slaves in Confederate-controlled territories — but it gave them an overwhelming incentive to do whatever they could to expedite Union victory. And it gave them the assurance that Union armies would free them if the soldiers of “Father Abraham” were victorious.

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The reactions were quite mixed. Attorney General Edward Bates approved. So did Secretary of War Edwin Stanton. Treasury Secretary Salmon Chase — a free soil leader who had been one of Lincoln’s rivals for the 1860 Republican nomination — recorded in his diary that he would give the measure “cordial support” but advised that emancipation might be better carried out at the discretion of commanders in the field. (Stanton recorded a very different reaction by Chase, who, according to Stanton’s recollection, called the proclamation a “measure of great danger”).

Others took a more measured, politically informed approach. Postmaster General Montgomery Blair said he feared that the proclamation would trigger a huge anti-Republican backlash in the fall elections. Most decisively, William Seward counseled Lincoln to wait until a military victory provided an occasion to issue the proclamation from the high ground of battlefield success.

Lincoln took Seward’s advice. He waited until after a Union victory — at Antietam, in September — to make the plan public, and he didn’t put the proclamation into effect until after elections. In the months in between, he would use all his gifts of persuasion — and deception — to soften up public opinion in the North, lest Republicans lose at the ballot box any chance to maintain the momentum of their antislavery mission.

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Richard Striner, a history professor at Washington College, is the author of “Lincoln and Race.”