Federalism, the division of power between state and central governments, was the most novel doctrine to emerge from the Constitutional Convention. ‘Federalism’ explains how it embraced a contradiction, imperium in imperio, a sovereignty within a sovereignty. This logical inconsistency—classical theory assumed that governmental sovereignty was indivisible—could be explained only by another innovation, popular sovereignty, which vested ultimate power in the people. Federalism has proven to be a highly malleable scheme for accommodating the demands of a diverse society and a dynamic economy. What began in 1787 as a partnership of equal governments became a powerful national government two centuries later, with widespread authority to safeguard (or threaten) liberty for its citizens.