No Roman emperor had a greater impact on the modern world than did Constantine. The reason is not simply that he converted to Christianity, but that he did so in a way that brought his subjects along after him. Indeed, this major new biography argues that Constantine's conversion is but one feature of a unique administrative style that enabled him to take control of an empire beset by internal rebellions and external threats by Persians and Goths. The vast record of Constantine's administration reveals a government careful in its exercise of power but capable of ruthless, even savage,...