The way he very broadly sums it up was the South didn't have a reason to revolt until Lord Mansfield's ruling on slavery in the empire sparked fear and uncertainty of slavery's future in the South. That's what united the Northern and Southern states against the British. The book also goes on the explain the critical role slavery played in molding our constitution because it was the most contentious piece. It suggests that once the states reached a compromise on slavery, the rest of the pieces fell into place.
I think that's a better characterization than "The US united to commit genocide." My story at least tries to flesh out how the interests of real humans leads them to commit atrocities. Your obtuse characterization paints them as villains with the depth of a comic book character. You will never learn from history if you paint people like comic book characters, because they'll never resemble the complex real people you're trying to compare them to.