Explanation: In the US Civil War, the core of the conflict was over the rights of Black folk as human beings, with the secessionist Confederacy being in support of race-based slavery and white supremacy as the literal foundation of their government and society, and openly and proudly so; and the Union, the democratically elected pre-war government, eventually, reluctantly, coming around to the idea of abolishing slavery and affording Black folk equal rights. Some Unionists were more based on this issue than others.
When Black soldiers began to be employed by the Union after the first year of the war, the slaver Confederates were shocked and appalled, and often objected in ways that were sometimes hilariously pathetic, and sometimes... tellingly gruesome and cruel. This is the case of the former - when captured Confederate soldiers learned that they were going to be escorted as PoWs by a unit of Black troops, they protested to Union General George Thomas. He told them to fuck off, but eloquently, as seen in the meme. The slaver scum meekly submitted after being put in their place, like slavers generally do when they aren't separated from their victims by chains and whips.
For bonus points, the unit of Black troops were almost certainly not the only troops he could spare at the time, lmao.