Ta-Nehisi Coates discusses the reckoning of Civil War history as a black man schooled in the non-violence techniques of the Civil Rights era.
On a more specific level, the Civil War presents something of an ideological challenge. Old school nationalists may well identify with black men literally fighting for their freedom–but the fact that those soldiers were doing so under the American flag, and are an exceptional chapter in the American martial tradition presents a problem. Old school integrationists, can except that latter portion, but the fact that 180,000 black people took up guns presents a deep challenge to the notion that black freedom was achieved nonviolently. On the contrary freedom was most literally achieved through the reception and infliction of horrific violence, and completed through the utter rejection of that violence. Perhaps that’s the point.