The introduction outlines the conventional narrative that this book seeks to question: The idea that the allocation of the legal right to wage war only to sovereign states, penned by a humanitarianly minded Gentili and implemented in practice through the seventeenth century, became one of the core stabilizing factors of the new states-system in the aftermath of the cataclysmic Wars of Religion. It then lays out the book’s core argument along with its stakes for contemporary debates about the regulation of warfare in the international order.