BTW, yes, I'm aware I'm currently a day behind on the daily postings.
Susanna Michele Lee, author of Claiming the Union: Citizenship in the Post-Civil War South (2014) says:
The most important moment of the Civil War, in my judgment, occurred when enslaved people first escaped to Union lines and found refuge there. These moments, as they recurred over and over again, started a revolution. The United States initially pledged not to interfere with slavery in the Confederacy. Slaves who ran to Union lines, however, placed abolition on the wartime agenda. Soldiers and officers were confronted with a choice: they could allow slaves to stay in camp and accept their assistance in waging war against the Confederacy, or they could send them back to their masters and mistresses who would attempt to use them to support the Confederate war effort. Soldiers and officers allowed slaves in Union lines with General Benjamin Butler serving as the most well-known example. The Union armies then became forces of emancipation. Slaves provided the initiative that ultimately culminated in the Confiscation Acts, the Emancipation Proclamation, and the Thirteenth Amendment. Slaves’ efforts in running to Union lines and offering their services as laborers and as soldiers helped to ensure that the Union preserved in the war would be fundamentally transformed, recognizing not only black freedom, but also black citizenship. [my emphasis]
The "starting a revolution" phrase isn't just rhetoric. When the Union declared the end of slavery its goal in the Emancipation Proclamation - technically, it didn't includes slaves in states like Missouri remaining in the union - it became a war to overturn the social and economic system in the South. In the language of the time, derived from the experience of the French Revolutionary and Napoleonic wars, that converted the War for the Union from a conventional war into a revolutionary war.
The key role of African-Americans' initiative is important to remember. Because erasing them from the narrative of the Civil War is one of the main features of the Lost Cause/neo-Confederate version of the war. In which slavery had nothing to do with starting the war. And in which black people were the passive recipients of the gift of freedom from white soldiers.
Benjamin Butler after the war became a pro-Reconstruction Republican Senator. His son-in-law, Adelbert Ames, was the last Reconstruction Governor of Mississippi. And in my mind the greatest of Mississippi's Governors because of his intense commitment to democracy.