Looking back over our history, our arrival at this July Fourth - free, secure and prosperous and celebrating Independence Day with fireworks and flags - now seems like an inevitable historical progression. But the American republic was never inevitable, and it was probably least inevitable 229 years ago on the day when the Founding Fathers signed it into existence, as historian David McCullough usefully reminds us in his marvelous new book on that perilous and pivotal year, "1776." What is justly best known from the Declaration of Independence is Thomas Jefferson's brilliant summation of the American ideal, "We hold these...