In the late 1990s, there was considerable historiographical debate about this topic among Civil War scholars. The two leading historians who tackled this topic was Ira Berlin and James McPherson. Dr. Berlin held that slaves freed themselves primarily through "self-emancipation." Her augment centered on the waffling of the Lincoln administration and certain army commanders when dealing with runaway slaves. Furthermore, she argued that without the efforts of African-Americans, the administration and army commanders may not have felt the need to deal with the runaway slave or contraband question.

Dr. McPherson on the other hand contended that, while Dr. Berlin's position had merit, the argument could only be taken so far. Essentially, McPherson argued that at the end of the day, Lincoln was commander-in-chief and had a huge role in the conduct of the army in regards to runaway slaves. Furthermore, Lincoln took several executive actions, like the Emancipation Proclamation, to encourage efforts of self-emancipation. Furthermore, it was Lincoln's political efforts to force through constitutional amendments that enshrined and maintained the freedom that African Americans worked so hard to attain for themselves.

Personally, I believe both scholars are right to varying degrees and present well-supported arguments that force us to reevaluate the traditional interpretation of Lincoln as the Great Emancipator. (Sources: McPherson, James M. "Who Freed the Slaves?." Proceedings of the American Philosophical Society (1995): 1-10; James M. McPherson. Drawn with the Sword: Reflections on the American Civil War. New York: Oxford University Press, 1996; Berlin, Ira. "Who Freed the Slaves? Emancipation and Its Meaning." Major Problems in the Civil War and Reconstruction (1997); Doris Kearns Goodwin, Team of Rivals: The Political Genius of Abraham Lincoln)

P.S. I stated that this was a trend of historiography in the late 1990s, it also appears to be making a come-back. See Williams, David. I Freed Myself: African American Self-Emancipation in the Civil War Era. Cambridge University Press, 2014.