Historical reports about Grinnell and Rousseau often state that the assault was because Mr. Grinnell insulted Rousseau personally and insulted the State of Kentucky. That is true, but that is not the whole story.
The Congressional Globe recorded debates that took place in the House and the Senate in 1833-1873. Those volumes are available online.
I looked through the Globe for the conversations that apparently took place on or around June 14, 1866. I did not see J. B. Grinnell’s recorded there, but I did find speeches of both Mr. Rousseau and Mr. Grinnell from earlier in the year. Grinnell’s speech, in which he does insult Rousseau was from February 5, 1866. Mr. Rousseau gave a speech on February 3, 1866.
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Here is an excerpt from the Rousseau speech:
“Sir, as I remarked before, the judicial and other officers of Kentucky, if they refuse to violate their oaths in obedience to the behests of the agents of this bureau, may be arrested and punished by them, this bureau acting as an appellate court on all subjects, while appeals lie from nowhere.
If you get on the cars with your wife and daughter, and if there be a spare seat, and a drunken negro comes forward to take it, and you ask him if he pleases to move a little further off, and takes a notion that he will not do it, and should report to the bureau that because he was a negro he was not allowed to take the seat, this Freedmen’s Bureau may at once arrest you and your daughter, and fine and imprison both. I say this bill authorizes that thing, and I defy any one of its friends to successfully combat that position. If you go to a theater in a place where this Freedmen’s Bureau is established, and, not because they are negros, but because they are unfit and ignorant persons, they are told the have no right to go and take seats with your family, and you prevent it, the bureau may arrest and imprison you. If a judge decides that a negro cannot be sworn in a cause being tried in his court, under the laws of a State which he has sworn to administer, why, sir before that decision is cold upon his lips they may arrest and take him off to the agent of the bureau and punish him as stated.”