Got this image off of an incredible Library of Congress site filled with images of 1861. William Waud drew these Charleston slaves mounting a cannon in Ft. Sumter in March of 1861. I'm sure the slaves were rented from the citizens of Charleston by the Federal government. This was before the Yankees KNEW that "THE REASON FOR THE WAR" was TO FREE THE SLAVES.
http://www.loc.gov/rr/print/list/picamer/paCw1861.html