I was in high school when read any books dealing with this. Pardon my ignorance, Nate; who are the other two men in the cartoon?
The guy on the right is a caricature of an Irish immigrant...Nast always portrayed them with a Simian style & associated with mob violence. There is also a caricature on the right of an affluent NY business man who is buying votes.
@StevenACole asked
Were there ever any USCT prisoners of war? Forrest captured USCT troops in several battles including Fort Pillow. They were sent to work on fortifications at Mobile and had a much higher survival rate than the white soldiers captured at Fort Pillow.
As to the article in the original OP, General W T Sherman did not participate in the Congressional Investigation of Fort Pillow of the later Congressional Investigation of the KKK. His brother, Senator John Sherman was on the second committee.
Four days after the battle, General Sherman was charged by Sec of War Stanton to conduct a military investigation of Fort Pillow. He delegated the task to General Brayman who collected affidavits from wounded soldiers from the battle in ILL. However, he was happy to turn over the task to the Joint Subcommittee on the Conduct of the War when Ben Wade showed up.
So the supposed Sherman quote : "We are here to investigate, try and convict" is fake. However he did write this in his memoirs:
The massacre at Fort Pillow occurred April 12, 1864, and has been the subject of congressional inquiry. No doubt Forrest's men acted like a set of barbarians, shooting down the helpless negro garrison after the fort was in their possession; but I am told that Forrest personally disclaims any active participation in the assault, and that he stopped the firing as soon as he could. I also take it for granted that Forrest did not lead the assault in person, and consequently that he was to the rear, out of sight if not of hearing at the time, and I was told by hundreds of our men, who were at various times prisoners in Forrest's possession, that he was usually very kind to them. He had a desperate set of fellows under him, and at that very time there is no doubt the feeling of the Southern people was fearfully savage on this very point of our making soldiers out of their late slaves, and Forrest may have shared the feeling.
Grant handled the subject in his memoir by quoting Forrest battle report.
Congress got back into the Fort Pillow business at least two more times. The first occurred in 1879 after Forrest death. The occasion was the election of J R Chalmers to the US House from a district in Mississippi. Some GOP members did not want to seat him and while not a full blown investigation the issues were revisited. Chalmers produced Charles Fitch's statements to defend himself.
Fort Pillow was again on the minds of Congress in 1899 when some members thought that the US military was guilty of similar actions in the Filipino War. The 1864 investigation was entered into the record again and the old arguments about what really happened were reopened.