THE AMERICAN effort to achieve independence from Britain was not expected to succeed, but it did, with some timely help from friends abroad. The new country’s grand experiment in representative self-government was not given much of a chance, either, but it has survived. The idea of a nation — not an empire, but a nation — melded from many nationalities, ethnic groups and religious faiths was inconceivable to most of the world, but it has been realized over the 239 years since independence was boldly declared in Philadelphia.