This chapter develops arguments about what statesmen expected from international law in the nineteenth century. Starting with an evaluation of an arms control agreement often decried as evidence of the failure of international law, the 1856 Black Sea Neutralization Treaty, institutions and sources of international law are set out. Next, the functions of international law are explored, developing arguments about the roles of law in diplomacy. Finally, the chapter describes enforcement of international law and the implications of enforcement through war. Nineteenth century expectations about international law provide rich insights into beliefs about the future of war, a largely unexplored avenue for interpreting pre-1914 diplomacy.