The American republic has endured for well over two centuries, but over the past 50 years, the apparatus of American governance has undergone a radical transformation. In some basic respects—its scale, its preoccupations, even many of its purposes—the U.S. government today would be scarcely recognizable to Franklin D. Roosevelt, much less to Abraham Lincoln or Thomas Jefferson.

What is monumentally new about the American state today is the vast empire of entitlement payments that it protects, manages and finances. Within...