The ancient Greek world was permeated with international law, the binding rules of behavior that Greeks across the Mediterranean followed and enforced for centuries. But why was international law effective in this world? Laws of All the Greeks offers an original answer rooted in contemporary social theory, presenting a definitive account of ancient Greek international law while delving into the social and psychological foundations that made it potent and durable. Challenging recent claims that international law did not exist in the Greek world, Laws of All the Greeks makes a robust case not only for its existence, but for the importance of taking ancient Greek international law as an historical subject in the first place. It provides an updated history of select elements of that law, including the right to private self-help; judicial treaties known as symbola; and piracy. Combining the findings of recent scholarship on Greek history, identity, and social networks, novel readings of documentary and literary sources, and the lessons of sociology and psychology, Jesse James demonstrates how the webs of identity binding the Greek world together affected both the rules of international law and their effectiveness at guiding legal and economic behavior. This study thus complicates and enriches prevailing approaches that emphasize formal institutions, structural features, or economic rationalism, offering instead a model for applying legal sociology to the historical study of international law.