Here's Paul Johnson's interesting take on the coming into being of America, in his "A History of the English People":-
By the time of Independence, slaves formed nearly one fifth of the [North]American population. The anomaly did not go unnoticed. If, as the patriots contended, nobody need be bound by laws they have not consented to themselves, or through their representatives, where did the slaves stand? The question was asked vociferously by many New England idealists. . . . Early and vigorous efforts were made from New England to get the transportation trade, at least, suppressed. No attempts were made to justify slavery on grounds of morality and logic. But the arguments for the economic necessity of slave-labour were regarded as unanswerable.
So, the English gave birth to a noisy, noble and flawed offspring, lavishing on it their traditional christening-gifts of idealism and hypocrisy. The taste for violence from which the English had always wished to free themselves - and were at last beginning to do so - passed across the Atlantic, where it struck deep and constitutional roots. England also handed on to America the birthright of the chosen race, while she herself assumed a secular role, increasingly shaped by the necessities and moral problems of empire, the 'white man's burden'.