Prior to the 13th amendment, it was agreed on all hands that the federal government had no power to regulate or abolish slavery in the states in which it existed. The Republican party’s agenda was to prevent the introduction of slavery into the territories acquired through the Mexican-American War, to prevent its introduction into new states admitted into the Union, to abolish slavery in the District of Columba, and to regulate or even prohibit the interstate and intercoastal slave trade. These are all powers firmly delegated to the federal government by the Constitution.
Lincoln was not an Abolitionist when the war began; his concern was to preserve the Union, not to end slavery. He was thoroughly racist, though not unusually so for his time; he dabbled in preposterous schemes like resettling the entire Black population of the United States, or at least the entire enslaved population, “back” in Africa, a continent completely foreign to the entire Black and enslaved population of our country. His, and most Republicans’, long-term plan was to restrict and regulate slavery and let it die of its own accord.
But he could not simply abandon the Republican platform in order to preserve the Union. He had been elected with a mandate to fulfill that platform. Given the Slavers’ Rebellion and its challenge to the authority of the federal government, his executive authority was at a zenith. He drew up an army adequate to enforce federal law in states overrun by seditious criminals; he went so far as to suspend the right of habeas corpus in still-loyal areas that were nevertheless infested with anti-American agents. These were war measures.
Likewise, as of January 1, 1863, he imposed an additional war measure: he emancipated, by Presidential decree, everyone held in bondage in areas then in revolt. Just as the President does not have the power to suspend habeas corpus except in extreme emergencies, the President did not have the power to emancipate slaves — except in extreme emergencies. Lincoln never contended that he had the power to emancipate slaves except as a war measure. Thus, he did not emancipate slaves in territories loyal to the Union, because such an emancipation would not advance the goal of re-introducing federal authority into areas then in rebellion. Lincoln remained within his legal powers despite having become an Abolitionist.
Those held as slaves in Kentucky and Maryland, and “seceded” areas that had been retaken by federal armies, were not emancipated. Thus, it’s sometimes said that the Emancipation Proclamation did not free any slaves.
But did the Emancipation Proclamation “free any slaves”? Yes: as a matter of law, it freed everyone held as a slave in territories still in revolt. It did not stay the whip, but the law alone never does: it takes force to enforce the law. Enslaved people fled to liberty and federal armies brought liberation, but they brought it now with the authority vested in the President by the Constitution — by the people.