He didn't mention that the enlightenment ideals on which ththe US based its constitution were formed in Europe, and France in particular. Britain's Thomas Payne - who was so disliked by the British establishment - helped formulate this in the US However I will need to re-read the essay to give it more thought before commenting further.
Also there's so much uninhabited space in the USA - many migrants tried to make the trek to the West Coast. But they met problems too - the American Indians, still trouble festering there. Same happened in Australia with the Aborigines, and S. Africa.