This book challenges the conventional story of sovereignty told in the field of International Relations. The conventional story holds that sovereignty has “traditionally” entailed unfettered rights to autonomous self-government and freedom from external intervention. Only in recent years, the tale goes, have these indefeasible rights been challenged by ideas that sovereigns should be accountable for the protection of their populations from mass atrocities. Present day concepts such as the “responsibility to protect” are thus framed as radical departures from the centuries-old meaning of sovereignty. This conventional story is wrong. The book offers a new story of sovereignty. It argues that sovereignty has been understood to entail varied and evolving responsibilities for the protection of populations since it first emerged in early modern Europe, and that sovereigns have been held to be accountable to God, to “the people,” and to the international society of states for carrying out these responsibilities. It demonstrates that the supposed “traditional,” non-interventionist meaning of sovereignty was only firmly established by states for the first time in the twentieth century, and then only for a few decades before again being challenged by claims that sovereigns are responsible and accountable for the protection of their populations. The book traces the relationship between sovereignty and responsibility, from the early modern period through to today, and tells a new history that has profound implications for present day debates.