I think you both can be right: "The US united to commit both genocide and slavery."

When reading the parent post, I too thought it left out slavery. (Understandable, in a short comment.) As I understand, Europeans generally were against slavery, not heeding intellectuals in favor of it. (In part probably because in preceding centuries they'd been its victims more often than not.) Nation states are about promoting a certain culture to dominate its territorial boundaries, and central to US culture is the invention of modern racism, to make its slave society back then robust.