In a time of great national division, a time of threats of resistance and counterthreats of suppression, a controversial president takes drastic measures to rein in his critics, citing national interest, national security, and his obligations as chief executive. If this seems familiar in our current moment of intense political agitation, that is all the more reason to attend to Thomas Mackey’s gripping, learned, and eminently readable account of the Civil War–era case of Clement L. Vallandigham, an Ohio congressman arrested for campaigning against the war and President Lincoln’s policies. In Mackey’s telling, the story of this prominent “Copperhead,” or Southern sympathizer, illuminates the problem of internal security, loyalty, and disloyalty faced by the Lincoln administration during wartime—and, more generally, the problem of determining the balance between executive power and tyranny, and between dissent and treason. Opposing Lincoln explores Vallandigham’s opposition not only to Lincoln and his administration but also to Lincoln’s use of force and his executive orders suspending habeas corpus. In addition to tracing Vallandigham’s experiences of being arrested, tried, convicted by military commission instead of civilian courts, and then banished from the United States, this historical narrative introduces readers to Lincoln’s most important statements on presidential powers in wartime, while also providing a primer on the wealth of detail involved in such legal and military controversies. Examining the long-standing issue of the limits of political dissent in wartime, the book asks the critical historical question of what reasonable lengths a legitimate government can go to in order to protect itself and its citizens from threats, whether external or internal. The case of Clement Vallandigham is, Mackey suggests, a quintessentially American story. Testing the limits of dissent in a political democracy in wartime, and of the scope and power of constitutional government, it clarifies a critical aspect of the American experience from afar.