Yes, because it was during a civil war and he claimed it to fall under his war powers:
The Civil War transformed Lincoln from a president into the constitutional commander in chief of the army and navy. What that meant, exactly, was far from clear. But that status might also give him yet another route toward emancipation. International law recognized that, in time of war, military commanders had the authority to suspend the normal operations of civil law and rule by decree. Only one American general—Andrew Jackson—had actually ever done this on American soil. But there had also never been a civil war in America before. And so Lincoln began gradually flexing his “war powers”—he suspended the writ of habeas corpus, he called up armies of volunteers into federal service, and he imposed a blockade of the Confederacy.
Of course, at each point, Taney and the Supreme Court hotly contested Lincoln’s use of these “war powers.” And this made him leery of pressing the “war powers” to the point of emancipating the South’s slaves. But by the summer of 1862, the military aspect of the war was going very badly. Relying on the labor of its slaves, the Confederacy was able to field armies that could easily hold their own against Lincoln’s armies. And his own generals—chief among them George B. McClellan, the commander of the Army of the Potomac—were politically unsympathetic to emancipation and sulky in taking orders from Lincoln. And to make matters worse, Delaware turned down the buy-out plan. If Lincoln waited longer, he might have victorious rebel armies surrounding Washington; or he might have mutinous generals threatening to seize the government for themselves. We “must change our tactics or lose the game,” Lincoln announced, and on July 22, 1862, Lincoln read to his cabinet a draft of an emancipation proclamation, threatening to decree the freeing of the slaves as a “fit and necessary war measure for suppressing” the rebellion. When McClellan and his army finally defeated the Confederates at Antietam in September, Lincoln published the proclamation and gave the Confederates 100 days to end the rebellion or the emancipation would take effect. The 100 days came and went without any repenting on the part of the Confederates, and on January 1, 1863, Lincoln signed the Emancipation Proclamation into law.
Into military law, that is. Lincoln had no more civil authority as president to emancipate slaves in 1863 than he had had at the beginning of the presidency, and every lawyer in the country knew it. This is why the proclamation did two very peculiar things: first, it based its emancipating authority strictly on Lincoln’s “power in me vested as Commander-in-Chief . . . in time of actual armed rebellion.” Second, it limited the reach of emancipation only to the slaves in “the States and parts of States . . . this day in rebellion against the United States.” The four slave states that had not joined the Confederacy (Delaware, Maryland, Kentucky, and Missouri) were not “in rebellion” and so Lincoln’s “war powers” had no reach over them.
