Before the Emancipation Proclamation, questions arose concerning the legal status and the practical treatment of slaves who escaped across Union military lines.  The Confederacy insisted that the Union abide by the Fugitive Slave Act of 1850 and return the slaves to their masters.  In May 1861, Union general Benjamin Butler refused to comply, and labeled the runaway slaves, whom the Confederacy considered property, as “contraband of war” (i.e., seized property).  But for nearly a year there was no official policy; consequently, some Union commanders offered shelter to fugitive slaves, while others turned them away. 

Gradually, Congress and President Abraham Lincoln began to chip away at the institution of slavery.  In August 1861, Congress passed a law declaring that runaway or captured slaves could not be returned to their masters if their masters used them for military purposes.  In the same month, though, the president rescinded an emancipation order of General John C. Fremont in Missouri.  Lincoln was anxious to keep the Border States (slave states that did not secede) in the Union.  Several times over the following months, he did urge the Border States to emancipate their slaves themselves.  

In 1862, the federal government began to enact more sweeping measures against slavery.  In March, Congress prohibited, under threat of court-martial, the return of all slaves to their masters, not just those forced to aid the Confederate military.  The next month, Congress abolished slavery in the District of Columbia, with financial compensation to former slave owners (about $300 per slave).  In June, Congress banned slavery in the territories, without compensation to former slave owners.

In July 1862, President Lincoln informed his cabinet that he planned to issue a proclamation, based on his constitutional authority as commander in chief, emancipating the slaves in Confederate territory.  Secretary of State William Henry Seward convinced him to wait until after a major Union victory.  Therefore, following the Union victory at Antietam on September 17, Lincoln announced the Preliminary Emancipation Proclamation.  Under its terms, if the Confederacy did not surrender by January 1, 1863, the president would free all the slaves in Confederate territory.  If the Confederate states did surrender, then their slaves would not be freed.  The Confederacy did not accept the offer, and the Emancipation Proclamation went into effect as stipulated.

In recent years, one of the major debates among Civil War historians has been whether President Abraham Lincoln or the slaves themselves were more responsible for emancipation.  Both sides on the question realize that the actions of Lincoln, the slaves, Congress, abolitionists, and others were contributing factors in the emancipation of slaves.  The dispute is over which was the key element in emancipation.

One side observes that Lincoln had consistently opposed slavery as immoral, and once adopting a policy of emancipation in 1862, he remained firmly committed to it, despite intense and vocal opposition.  These historians characterize President Lincoln's role as vital in forging a coalition of Unionists from diverse and sometime antagonistic groups, guiding public opinion toward emancipation, insisting upon unconditional surrender, and transforming the Union military into what became partly an army of liberation as it marched across the South conquering more territory.  

Critics point out that the Emancipation Proclamation was limited to Confederate-held territory (not the Border States or Union territory in Southern slave states captured before January 1, 1863).  These historians emphasize slaves as the ones who took the risks for emancipating themselves and their fellow slaves.  Even before the Emancipation Proclamation, the "contrabands" left behind their old lives to cross Union lines and often contributed to the Union cause as guides, spies, laborers, and (from 1863) soldiers and sailors.  Their actions helped convince white Unionists (even racist ones) that emancipation would greatly benefit the Union military cause.