As a last-ditch effort, the Confederacy armed black slaves to fight for their cause. In addressing his army, 140th Indiana Regiment, Lincoln saw the duplicity of this thinking.
3/17/1865...There are but few aspects of this great war on which I have not already expressed my views by speaking or writing. There is one-the recent effort of our erring brethren, sometimes so called, to employ the slaves in their armies. The great question with them has been 'will the Negro fight for them?' They ought to know better that we; and, doubtless, do know better that we. I may incidentally remark, however, that having, in my life, heard many arguments...intended to show that the Negro ought to be a slave, that if he shall now really fight to keep himself a slave, it will be a far better argument why he should remain a slave than I have ever before heard. He, perhaps, ought to be a slave, if he desires it ardently enough to fight for it. Or, if one out of four will, for his own freedom, fight to keep the other three in slavery, he ought to be a slave for his selfish meanness.
I have always thought that all men should be free; but if any should be slaves it should be first those who desire it for themselves, and secondly those who desire it for others. Whenever I hear anyone arguing for slavery I feel a strong impulse to see it tried on him personally. FT 640-1