In the decades of the early republic, Americans debating the fate of slavery often invoked the specter of disunion to frighten their opponents. As Elizabeth Varon shows, "disunion" connoted the dissolution of the republic--the failure of the founders' effort to establish a stable and lasting representative government. For many Americans in both the North and the South, disunion was a nightmare, a cataclysm that would plunge the nation into the kind...